Endorses fish or rump
I believe the answer is:
backside
'rump' is the definition.
('backside' can be a synonym of 'rump')
'endorses fish' is the wordplay.
'endorses' becomes 'backs' ('back' can be a synonym of 'endorse').
'fish' becomes 'ide' (I've seen this before).
'backs'+'ide'='BACKSIDE'
'or' is the link.
(Other definitions for backside that I've seen before include "Rear" , "Bottom" , "Seat" , "Behind" , "Hind part" .)