Discover part of a fish with a red back (4)
I believe the answer is:
find
'discover' is the definition.
(I know that discover can be written as find)
'part of a fish with a red back' is the wordplay.
'part of a fish' becomes 'fin' ('fin' is part of 'fish').
'with' means one lot of letters go next to another.
'back' indicates one should take the final letters (the letter at the back).
The final letter of 'red' is 'd'.
'fin'+'d'='FIND'
(Other definitions for find that I've seen before include "Encounter, locate" , "Realise" , "Discover by chance" , "Track down" , "Acquisition" .)