Former lover knocks one leg, creating swelling (9)
I believe the answer is:
expansion
'swelling' is the definition.
(thesaurus)
'former lover knocks one leg' is the wordplay.
'former lover' becomes 'ex' (an ex-lover).
'knocks' becomes 'pans' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'leg' becomes 'on' (cricket term, fielding positions).
'ex'+'pans'+'i'+'on'='EXPANSION'
'creating' is the link.
(Other definitions for expansion that I've seen before include "Process of increasing in size, volume etc" , "increase in capacity" , "Enlargement" , "fatter" , "Increase in volume" .)