Sabotage ruined men's pants (9)
I believe the answer is:
undermine
'sabotage' is the definition.
('undermine' can be a synonym of 'sabotage')
'ruined men's pants' is the wordplay.
'pants' indicates an anagram (UK informal term for 'nonsense' or 'rubbish').
'ruined'+'men'='ruinedmen'
'ruinedmen' is an anagram of 'UNDERMINE'.
(Other definitions for undermine that I've seen before include "Erode the base or foundation of" , "Sabotage; subvert" , "Erode foundations" , "Weaken insidiously" , "Weaken, sap" .)