British League's problem affecting one member (6)
I believe the answer is:
'problem affecting one member' is the definition.
(swelling on a foot)
'british league's' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'league' becomes 'union' (I've seen this before).
(Other definitions for bunion that I've seen before include "Painful swelling at base of big toe" , "Swelling on the foot" , "Painful swelling of the big toe joint" , "Foot fault" , "Swelling at base of toe with thickened skin" .)