Spoil one’s twins (6)
I believe the answer is:
impair
'spoil' is the definition.
(I know that spoil can be written as impair)
'one's twins' is the wordplay.
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'twins' becomes 'pair' (a pair of twins).
'im'+'pair'='IMPAIR'
(Other definitions for impair that I've seen before include "blunt" , "Make worse or less effective" , "Weaken, damage" , "Harm" , "Cause weakening of, damage" .)