I am to get two to spoil (6)
I believe the answer is:
impair
'to spoil' is the definition.
(synonyms)
'i am to get two' is the wordplay.
'i am' becomes 'I'm' (contraction).
'to get' is a charade indicator (letters next to each other).
'two' becomes 'pair' (pair is a kind of two).
'im'+'pair'='IMPAIR'
(Other definitions for impair that I've seen before include "Make worse or less effective" , "Hobble" , "Damage or handicap" , "Harm" , "Damage or injure" .)