Pressure swelling when chubby (5)
I believe the answer is:
plump
'chubby' is the definition.
(I know that chubby can be written as plump)
'pressure swelling' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'swelling' becomes 'lump' (synonyms).
'p'+'lump'='PLUMP'
'when' acts as a link.
(Other definitions for plump that I've seen before include "Fat - set down unceremoniously" , "Choose; chubby" , "Tubby; opt (for)" , "Chubby, fat" , "Opt - round" .)