Fish from Aintree luncheonette (3)
I believe the answer is:
eel
'fish' is the definition.
(eel is a kind of fish)
'from aintree luncheonette' is the wordplay.
'from' indicates the answer is hidden within the clue.
'EEL' can be found hidden inside 'aintree luncheonette'.
(Other definitions for eel that I've seen before include "Snaky fish" , "Serpentine fish" , "Slippery person" , "Teleost fish" , "Creature like conger or moray" .)