Snack service, one in wrapper ready for delivery (6)
I believe the answer is:
sarnie
'snack' is the definition.
(informal term for a sandwich)
'service one in wrapper ready for delivery' is the wordplay.
'service' becomes 'RN' (abbreviation for Royal Navy).
'one' becomes 'I' (Roman numeral).
'in' is an insertion indicator.
'wrapper ready for delivery' becomes 'SAE' (self-addressed stamped envelope).
'rn'+'i'='rni'
'rni' put into 'sae' is 'SARNIE'.
(Other definitions for sarnie that I've seen before include "Sandwich (informal)" , "Sandwich (slang)" , "Familiar form of snack" , "Popular bread-based snack (colloq)" , "no classy fare" .)