Swelling comes down over the years (6)
I believe the answer is:
dropsy
'swelling' is the definition.
(dropsy is a kind of swelling)
'comes down over the years' is the wordplay.
'comes down' becomes 'drops' (dropping is a kind of coming down).
'over' is a charade indicator (letters next to each other).
'the years' becomes 'y' (abbreviation).
'drops'+'y'='DROPSY'
(Other definitions for dropsy that I've seen before include "fluid build-up" , "illness" , "medical condition" , "Accumulation of fluid in a part of the body - oedema" , "Oedema (old name)" .)