Spoil number for creature (6)
I believe the answer is:
marten
'for creature' is the definition.
(I've seen this before)
'spoil number' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'number' becomes 'ten' (ten is a kind of number).
'mar'+'ten'='MARTEN'
(Other definitions for marten that I've seen before include "Weasel-like mammal of forests" , "Furry carnivore, maybe of pine woods" , "Weasel-like animal with thick soft fur" , "forest-dweller" , "Animal - rat men (anag)" .)