Long, thin fish returned for a kipper (7)
I believe the answer is:
sleeper
'kipper' is the definition.
(I've seen this before)
'long thin fish returned for a' is the wordplay.
'long thin fish' becomes 'eels' (I've seen this before).
'returned' says the letters should be written in reverse.
'for' is a charade indicator (letters next to each other).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'eels' in reverse letter order is 'slee'.
'slee'+'per'='SLEEPER'
(Other definitions for sleeper that I've seen before include "Express tears over one" , "Brace to support rail on track" , "film not successful at first" , "Form of transport" , "Timber beam" .)