Small swelling to decline (5)
I believe the answer is:
slump
'decline' is the definition.
(slump is a kind of decline)
'small swelling' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'swelling' becomes 'lump' ('lump' can be a synonym of 'swelling').
's'+'lump'='SLUMP'
'to' is the link.
(Other definitions for slump that I've seen before include "Sudden market drop" , "Sit badly" , "Sudden decline" , "Economic depression" , "Sudden fall as in share values" .)