Monkey will spoil second collection (8)
I believe the answer is:
marmoset
'monkey' is the definition.
(I know that marmoset is a type of New World monkey)
'will spoil second collection' is the wordplay.
'will' says to put letters next to each other (I've seen this in other clues).
'spoil' becomes 'mar' (synonyms).
'second' becomes 'mo' (both are short periods of time).
'collection' becomes 'set' (set is a kind of collection).
'mar' next to 'mo' is 'marmo'.
'marmo'+'set'='MARMOSET'
(Other definitions for marmoset that I've seen before include "Small monkey - Rome's mat (anag)" , "Small New World monkey" , "Turn morse mat over for the monkey" , "Arms me to (anag) - animal" , "Small South American monkey" .)