Pierce with a knife (4)
I believe the answer is:
stab
'pierce with a knife' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stab that I've seen before include "Plunge knife into" , "wound as Casca" , "Wound with pointed weapon" , "Attack with a knife" , "Thrust (weapon into)" .)