Support British nation (5)
I believe the answer is:
brace
'support' is the definition.
(brace is a kind of support)
'british nation' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'nation' becomes 'race' (I've seen this before).
'b'+'race'='BRACE'
(Other definitions for brace that I've seen before include "Pair - strut" , "Strengthening piece" , "Support; two" , "Tighten, strengthen" , "A pair as of game birds" .)