Damage, weaken (6)
I believe the answer is:
impair
This clue is a double definition.
'damage' is the first definition.
(I know that impair is a more specific form of the action damage)
'weaken' is the second definition.
(I've seen this before)
(Other definitions for impair that I've seen before include "'Mar, damage (6)'" , "Make worse or less effective" , "Weaken, damage" , "Damage or injure" , "Damage or handicap" .)