Briton takes turn as vanguard (5)
I believe the answer is:
scout
'vanguard' is the definition.
(thesaurus)
'briton takes turn' is the wordplay.
'briton' becomes 'Scot' (Scot is a type of Briton).
'takes' means one lot of letters goes inside another.
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'scot' going around 'u' is 'SCOUT'.
'as' acts as a link.
(Other definitions for scout that I've seen before include "One searches for and recruits talented people" , "Person on the lookout" , "Member of organisation for boys" , "Member of a youth movement" , "Soldier on a reconnaissance mission" .)