A form of fish food (3)
I believe the answer is:
roe
'food' is the definition.
(roe is a kind of food)
'a form of fish' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is present in the answer.
The remaining letters 're' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for roe that I've seen before include "Swift runner" , "Hoofed animal(s)" , "A mass of fish-eggs" , "Food from fish" , "Lobster's ripe ovary" .)