Marine creature enjoyed turning no good (6)
I believe the answer is:
dugong
'marine creature' is the definition.
(I've seen this before)
'enjoyed turning no good' is the wordplay.
'enjoyed' becomes 'dug' (slang).
'turning' is a reversal indicator.
'good' becomes 'g' (abbreviation).
'no' written backwards gives 'on'.
'dug'+'on'+'g'='DUGONG'
(Other definitions for dugong that I've seen before include "Sea beast" , "Marine creature" , "Sea cow - gun dog (anag)" , "Sea cow with forked tail" , "Marine mammal" .)