Go after earl with a sword (4)
I believe the answer is:
epee
'a sword' is the definition.
(epee is a kind of sword)
'go after earl' is the wordplay.
'go' becomes 'pee' (both can mean to urinate).
'after' says to put letters next to each other.
'earl' becomes 'e'.
'pee' after 'e' is 'EPEE'.
'with' is the link.
(Other definitions for epee that I've seen before include "Fencing weapon" , "Thin fencing sword" , "sharp-edged implement" , "piece of fencing" , "Fencer's weapon" .)