Quince's second part's in Hamlet - he's playing patriarch (10)
I believe the answer is:
methuselah
'patriarch' is the definition.
(thesaurus)
'quince's second part's in hamlet he's playing' is the wordplay.
I cannot quite see how this works, but
'second' could be 's' (common abbreviation) and 's' is present in the answer.
'part' could be 'lah' (lah is a kind of part) and 'lah' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for methuselah that I've seen before include "able to hold a lot of drink" , "a lot of bottle" , "Old man of the Bible" , "He lived 969 years (Gen.)" , "Old Testament patriarch (very old when he died!)" .)