Fish, part of free lunch (3)
I believe the answer is:
eel
'fish' is the definition.
(eel is a kind of fish)
'part of free lunch' is the wordplay.
'part of' says the answer is hidden in the clue.
'EEL' is hidden in the letters of 'free lunch'.
(Other definitions for eel that I've seen before include "A sea snake" , "Unagi, in a sushi bar" , "Slippery person" , "Slippery swimmer (may be jellied)" , "Moray or electric, for instance" .)