Volunteers lettuce as a dish (5)
I believe the answer is:
tacos
'a dish' is the definition.
(I have seen 'Mexican dish ' mean 'tacos' so perhaps 'dish' could also mean 'tacos')
'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 "Snacks" , "Coats on Mexican tortilla dishes" , "Mexican wraps" , "Mexican food" , "Foreign dish" .)