British PM once briefly in fight (6)
I believe the answer is:
battle
'fight' is the definition.
(I know that fight can be written as battle)
'british pm once briefly' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'pm once' becomes 'Attlee' (prime minister Clement Attlee).
'briefly' means to remove the last letter.
'attlee' with its last letter removed is 'attle'.
'b'+'attle'='BATTLE'
'in' acts as a link.
(Other definitions for battle that I've seen before include "Lengthy struggle" , "Armed conflict" , "Sussex town" , "Skirmish" , "Sustained conflict" .)