Alternative headgear used by British union members, perhaps? (11)
I believe the answer is:
brotherhood
'union members perhaps?' is the definition.
(brotherhood can mean a trade union)
'alternative headgear used by british' is the wordplay.
'alternative' becomes 'other' (I've seen this before).
'headgear' becomes 'hood' (hood is a kind of headgear).
'used by' is a charade indicator (letters next to each other).
'british' becomes 'br' (abbreviation for British).
'other'+'hood'='otherhood'
'otherhood' after 'br' is 'BROTHERHOOD'.
(Other definitions for brotherhood that I've seen before include "association" , "Community" , "Fraternity" , "union" , "Feeling of close kinship" .)