Instruct British to avoid frontier (5)
I believe the answer is:
order
'instruct' is the definition.
(I've seen this before)
'british to avoid frontier' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'to avoid' is a deletion indicator.
'frontier' becomes 'border' (I've seen this before).
'border' with 'b' taken away is 'ORDER'.
(Other definitions for order that I've seen before include "association" , "pattern" , "One of Great Lakes" , "See 20" , "Instruction - arrangement" .)