Damage or weaken (6)
I believe the answer is:
impair
I believe this clue is a double definition.
'damage' is the first definition.
(I know this)
'weaken' is the second definition.
(I've seen this before)
(Other definitions for impair that I've seen before include "Damage or injure" , "Reduce" , "Spoil, degrade" , "Cause weakening of, damage" , "Damage or handicap" .)