Firm after sign of appreciation supplies Mexican food (4)
I believe the answer is:
taco
'mexican food' is the definition.
(I've seen this before)
'firm after sign of appreciation' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'after' means one lot of letters go next to another.
'sign of appreciation' becomes 'ta' (I can't explain this - if you can you should give a lot more credence to this answer).
'co' put after 'ta' is 'TACO'.
'supplies' acts as a link.
Can you help me to learn more?
(Other definitions for taco that I've seen before include "Mexican filled tortilla" , "Filled rolled tortilla" , "Mexican food" , "Tortilla rolled round a filling" , "Folded and filled tortilla" .)