Fish some enjoyed immensely on reflection (3)
I believe the answer is:
ide
'fish' is the definition.
(I've seen this before)
'some enjoyed immensely on reflection' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'on reflection' says the letters should be written in reverse.
'edi' is hidden within 'enjoyed immensely'.
'edi' back-to-front is 'IDE'.
(Other definitions for ide that I've seen before include "Silver orfe" , "Fish of the carp family" , "Fish, the orfe" .)