Union card (4)
I believe the answer is:
club
This clue is a double definition.
'union' is the first definition.
(thesaurus)
'card' is the second definition.
(club is a kind of card)
(Other definitions for club that I've seen before include "Place to meet one's friends with a weapon" , "Staff - association" , "Iron or putter, say" , "Association - bludgeon" , "'Short stick, card suit and association (4)'" .)