Boyfriend’s first marriage a cause of pain (6)
I believe the answer is:
bunion
'a cause of pain' is the definition.
(type of painful swelling)
'boyfriend's first marriage' is the wordplay.
'boyfriend's first' becomes 'b' (1st letter of 'boyfriend').
'marriage' becomes 'union' (synonyms).
'b'+'union'='BUNION'
(Other definitions for bunion that I've seen before include "Painful swelling of the big toe joint" , "Something painful" , "Foot fault" , "Swelling at base of toe with thickened skin" , "Swelling on the foot" .)