Instruct bishop to leave frontier (5)
I believe the answer is:
order
'instruct' is the definition.
(to instruct is to give orders)
'bishop to leave frontier' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'to leave' is a deletion indicator.
'frontier' becomes 'border' (I've seen this before).
'border' with 'b' taken out is 'ORDER'.
(Other definitions for order that I've seen before include "Mr Speaker's plea" , "See 20" , "Direct - sequence" , "Command harmony" , "Direction; class; arrange" .)