Left the fish on show (7)
I believe the answer is:
portray
'show' is the definition.
(I've seen this before)
'left the fish on' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'the fish' becomes 'ray' (ray is a kind of fish).
'on' says to put letters next to each other.
'port'+'ray'='PORTRAY'
(Other definitions for portray that I've seen before include "Describe in words or make a picture" , "Paint a picture" , "Delineate" , "Play" , "sketch" .)