Tell British to abandon frontier (5)
I believe the answer is:
order
'tell' is the definition.
(synonyms)
'british to abandon frontier' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'to abandon' indicates named letters should be taken away.
'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 "Edict, command" , "Uniformity or fraternity" , "the Speaker often calls for" , "It could be the usual" , "Ruling" .)