In the rough sea a good long time (4)
I believe the answer is:
ages
'long time' is the definition.
(I've seen this in another clue)
'in the rough sea a good' is the wordplay.
'in' is an insertion indicator.
'the rough' indicates an anagram (a rough or broken ordering of the letters).
'a good' becomes 'g' (abbreviation).
'sea' with letters rearranged gives 'aes'.
'aes' enclosing 'g' is 'AGES'.
(Other definitions for ages that I've seen before include "A long time (colloq.)" , "Aeons" , "Sage (anag)" , "Yonks" , "Donkey's years" .)