Tom, perhaps, sent back old Mexican food (4)
I believe the answer is:
taco
'mexican food' is the definition.
(type of Mexican snack)
'tom perhaps sent back old' is the wordplay.
'tom perhaps' becomes 'cat' (a tom is a male cat).
'sent back' says the letters should be written in reverse.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'cat' written backwards gives 'tac'.
'tac'+'o'='TACO'
(Other definitions for taco that I've seen before include "Mexican dish of rolled tortilla with filling" , "Filled, fried tortilla" , "Set of three" , "Tortilla rolled round a filling" , "Mexican filled pancake" .)