The plump fish next to (6)
I believe the answer is:
chubby
'the plump' is the definition.
(thesaurus)
'fish next to' is the wordplay.
'fish' becomes 'chub' (chub is a kind of fish**).
'next to' becomes 'by'.
'chub'+'by'='CHUBBY'
(Other definitions for chubby that I've seen before include "Plumb and round" , "On the plump side" , "with some corporation?" , "Slightly fat" , "Podgy" .)