Weaken insidiously (9)
I believe the answer is:
undermine
'weaken insidiously' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for undermine that I've seen before include "Subvert or weaken gradually" , "Burrow below" , "Weaken insidiously" , "Erode the base or foundation of" , "Sabotage; subvert" .)