Marine animal’s nasty smell — in sea, mostly (6)
I believe the answer is:
sponge
'marine animal's' is the definition.
(thesaurus)
'nasty smell in sea mostly' is the wordplay.
'nasty smell' becomes 'pong' (pong can informally mean a bad smell).
'in' indicates putting letters inside.
'mostly' means to remove the last letter.
'sea' with its last letter taken off is 'se'.
'pong' placed inside 'se' is 'SPONGE'.
(Other definitions for sponge that I've seen before include "Porous substance found in the ocean" , "soap?" , "that lives in the sea" , "Porous mass in oceans" , "Marine creature - cadge" .)