Plump fish badly gutted (6)
I believe the answer is:
chubby
'plump' is the definition.
(I know that plump can be written as chubby)
'fish badly gutted' is the wordplay.
'fish' becomes 'chub' (chub is a kind of fish).
'gutted' suggests removing the centre (to gut something is to remove its insides).
'badly' with its centre removed is 'by'.
'chub'+'by'='CHUBBY'
(Other definitions for chubby that I've seen before include "Slightly fat" , "with too much fat" , "with some corporation?" , "Roundly plump" , "Overweight" .)