Sea creature's attempt to cut waste (6)
I believe the answer is:
dugong
'sea creature's' is the definition.
(species of sea cow)
'attempt to cut waste' is the wordplay.
'attempt' becomes 'go' (go is a kind of attempt).
'to cut' is an insertion indicator.
'waste' becomes 'dung' (dung is a kind of waste).
'go' placed into 'dung' is 'DUGONG'.
(Other definitions for dugong that I've seen before include "Sea beast" , "Sea cow - gun dog (anag)" , "Marine mammal" , "Marine creature" , "Sea cow with forked tail" .)