Fish -- a bit of food served up (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'a bit of food served up' is the wordplay.
'bit of food' becomes 'nut' (nuts are food).
'served up' is a reversal indicator.
'a'+'nut'='anut'
'anut' in reverse letter order is 'TUNA'.
(Other definitions for tuna that I've seen before include "swimmer" , "Skipjack --, blue-fin --" , "Popular food fish" , "Yellow-finned fish" , "Large fish for the strange aunt" .)