Fabulous writer of work on marine set-up (5)
I believe the answer is:
aesop
'fabulous writer' is the definition.
(writer of fables)
'work on marine set-up' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'on' is a charade indicator (letters next to each other).
'marine' becomes 'sea' (both can mean 'of the sea').
'set up' shows that the letters should be reversed in order.
'sea' back-to-front is 'aes'.
'op' after 'aes' is 'AESOP'.
'of' acts as a link.
(Other definitions for aesop that I've seen before include "Greek author, famous for his fables" , "Greek fabulist (Tortoise and the Hare)" , "very old writer" , "A pose of famed fable teller" , "his writings were fabulous" .)