Stout puts pressure on swelling (5)
I believe the answer is:
plump
'stout' is the definition.
(I've seen this before)
'pressure on swelling' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'swelling' becomes 'lump' (synonyms).
'p'+'lump'='PLUMP'
'puts' acts as a link.
(Other definitions for plump that I've seen before include "Chubby, rounded" , "stout" , "Corpulent" , "Not obese, just pleasantly rounded" , "Well-rounded" .)