Plump freshwater fish in reserve (6)
I believe the answer is:
chubby
'plump' is the definition.
(I know that plump can be written as chubby)
'freshwater fish in reserve' is the wordplay.
'freshwater fish' becomes 'chub' (I've seen this in another clue).
'in reserve' becomes 'by' (synonyms).
'chub'+'by'='CHUBBY'
(Other definitions for chubby that I've seen before include "Attractively plump" , "Podgy" , "roly-poly" , "Overweight" , "On the plump side" .)