Knuckle under (7)
I believe the answer is:
succumb
'knuckle under' is the definition.
(synonyms)
This is all the clue.
(Other definitions for succumb that I've seen before include "Fail to resist (temptation, perhaps)" , "Give in (to disease, etc)" , "Give in, yield" , "Surrender" , "Gives in under pressure, capitulates" .)