Why can a noun already used with a definite article "the" be used with a defining relative clause?
If it is already specific as suggested by the article "the", why can it be followed by a defining relative clause used to provide essential information that helps to identify it?
There are cases where a defining relative clause can follow a noun with either a definite or indefinite article.
The definite article the merely signals that there is a single identifiable intended referent; it does not indicate that the speaker or listener does not need any additional description whatsoever to identify it. In fact, any further limiting description in that noun phrase should be sufficient to preempt a which? question, but not necessarily a who? as the actual identity (name) of the referrent may not be known.
So, if one says
one would expect the intended listener to be aware of only one man that could possibly be meant, for example if there had just been a conversation discussing the imminent arrival of said man. One would not expect an response of Which man? but Who is he? is possible.
On the other hand if one says
one is providing the extra information to preempt a Which man? question, assuming that the listener might be confused by simply the man, seeing as perhaps both interlocutors are aware of multiple possible men who could show up at any time. Again, the question Who is he? would still be possible as his actual identity may still be unknown.
The Cambridge Grammar of the English Language has a quite extensive section on definiteness, the articles, identifiablility, etc. starting on p368.
Identifiability
The concept of identifiability expressed by the definite article is best understood in terms of pre-empting a question with which? Compare, for example:
[2]
i Where did you park the car?
ii The father of one of my students rang me up last night.
iii The first person to run the mile in under four minutes was Roger Bannister.
Example [i] illustrates the frequent case where the addressee can be assumed to be familiar with the referent of the definite NP: you have been driving the car and presumably know a good deal more about it than that it is a car – what colour and make of car it is, and so on. You thus don’t need to ask Which car?: you know which one I’m referring to. Familiarity of this kind is not, however, a necessary condition for the felicitous use of the. In [2ii], for example, I don’t say who the student is and so can’t expect you to know who the father is. Nevertheless, I have a particular student in mind and the property of being father of that student provides distinctive, hence identifying, information about the referent. It wouldn’t make sense, therefore, to ask #Which father of one of your students? (Which one of your students? would of course make sense, but the which question here relates to the embedded NP one of my students, which is indefinite.) The kind of identifiability signalled by the is thus of a relatively weak kind. This is further illustrated in [2iii], where the predicate provides stronger identifying information. The head nominal of the definite NP in subject function defines a unique individual, so again the question #Which first person to run the mile in under four minutes? would be incoherent. But you can recognise that the description can only be met by a unique individual without knowing who the individual is: it is the predicate in [iii] that says who he is. The indicates, therefore, that the head gives identifying information that pre-empts a which question, but the information expressed in the head certainly need not pre-empt a who question.