Command British to leave frontier (5)
I believe the answer is:
order
'command' is the definition.
(order is a kind of command)
'british to leave frontier' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'to leave' suggests deleting specific letters.
'frontier' becomes 'border' (I've seen this before).
'border' with 'b' removed is 'ORDER'.
(Other definitions for order that I've seen before include "Religious fraternity" , "Right arrangement" , "in which things are properly arranged?" , "Decree" , "Direct - sequence" .)