Painful swelling, not initially an association (5)
I believe the answer is:
union
'an association' is the definition.
(I've seen this before)
'painful swelling not initially' is the wordplay.
'painful swelling' becomes 'bunion' (I've seen this before).
'not initially' means to remove the first letter.
'bunion' with its first letter removed is 'UNION'.
(Other definitions for union that I've seen before include "Marriage or alliance" , "Workers' association - marriage" , "The USA in the USA" , "See 22" , "Equity, perhaps" .)