Old animal set about Mexican food (4)
I believe the answer is:
taco
'mexican food' is the definition.
(I've seen this before)
'old animal set about' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'animal' becomes 'cat' (I've seen this before**).
'set about' says the letters should be written backwards.
'o'+'cat'='ocat'
'ocat' back-to-front is 'TACO'.
(Other definitions for taco that I've seen before include "Filled rolled tortilla" , "Tortilla rolled round a filling" , "Filled, fried tortilla" , "Mexican dish of rolled tortilla with filling" , "Mexican filled tortilla" .)