Support British competition (5)
I believe the answer is:
brace
'support' is the definition.
(I know that brace is a type of support)
'british competition' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'competition' becomes 'race' (race is a kind of competition).
'b'+'race'='BRACE'
(Other definitions for brace that I've seen before include "Two of a kind" , "Support; two" , "Secure" , "Strengthening piece" , "Strengthener; pair" .)