An aphid, surprisingly, is something you can feed to a fish (7)
I believe the answer is:
daphnia
'something you can feed to a fish' is the definition.
'an aphid surprisingly' is the wordplay.
'surprisingly' indicates an anagram.
'an'+'aphid'='anaphid'
'anaphid' with letters rearranged gives 'DAPHNIA'.
'is' acts as a link.
(Other definitions for daphnia that I've seen before include "Water flea" , "Small freshwater creature" , "Minute crustacean; an aphid (anag.)" .)