Support service by Britain (5)
I believe the answer is:
brace
'support' is the definition.
(bracing is a kind of supporting)
'service by britain' is the wordplay.
'service' becomes 'ace' (ace is a kind of service).
'by' means one lot of letters go next to another.
'britain' becomes 'br' (abbreviation).
'ace' put after 'br' is 'BRACE'.
(Other definitions for brace that I've seen before include "Support - couple" , "Two of a kind" , "Tighten, strengthen" , "hand tool" , "Stanchion" .)