A weapon by association (4)
I believe the answer is:
club
I believe this clue is a double definition.
'a weapon' is the first definition.
(I've seen this before)
'association' is the second definition.
(I know that association can be written as club)
'by' acts as a link.
(Other definitions for club that I've seen before include "Stout stick, card suit and association" , "sort of sandwich" , "Kind of weapon or golf stick" , "Place to meet one's friends with a weapon" , "Association for people with common interest" .)