Weaken, spoil (6)
I believe the answer is:
impair
This clue is a double definition.
'weaken' is the first definition.
(I've seen this before)
'spoil' is the second definition.
(synonyms)
(Other definitions for impair that I've seen before include "Hurt or handicap" , "Spoil, degrade" , "Make worse or less effective" , "Damage or handicap" , "Reduce" .)