Crumble under pressure (4)
I believe the answer is:
wilt
'crumble under pressure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wilt that I've seen before include "Droop in the heat" , "Ave --; black --" , "fade" , "Decline" , "Lose strength and become limp" .)