Cain's eldest (5)
I believe the answer is:
enoch
'cain's eldest' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for enoch that I've seen before include "Man in Bible" , "___ Powell, Tory politician, d. 1998" , "Father of Methuselah" , "Cain's eldest son - Methuselah's father" , "Oldest city father" .)