After start of encounter, go for weapon (4)
I believe the answer is:
epee
'for weapon' is the definition.
(epee is a kind of weapon)
'after start of encounter go' is the wordplay.
'after' means one lot of letters go next to another.
'start of' says to take the initial letters.
'go' becomes 'pee' (both can mean to urinate).
The initial letter of 'encounter' is 'e'.
'e' next to 'pee' is 'EPEE'.
(Other definitions for epee that I've seen before include "Fencer's sword" , "Fencer's weapon" , "sports equipment" , "Fencing blade" , "Fencing sword similar to a foil" .)