Volunteers lettuce as Mexican food (5)
I believe the answer is:
tacos
'mexican food' is the definition.
(I've seen this in another clue)
'volunteers lettuce' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'lettuce' becomes 'cos' (cos lettuce is a kind of lettuce).
'ta'+'cos'='TACOS'
'as' is the link.
(Other definitions for tacos that I've seen before include "Filled tortillas" , "Coats on Mexican tortilla dishes" , "Mexican tortillas with various fillings" , "Foreign dish" , "Snacks" .)