Attempt to use a knife (4)
I believe the answer is:
stab
I believe this clue is a double definition.
'attempt' is the first definition.
(to have a stab is to attempt something)
'to use a knife' is the second definition.
(Other definitions for stab that I've seen before include "Jab" , "Knife thrust; attempt" , "Injure with knife" , "Strike with knife" , "Wound with blade" .)