Frontier having British command (6)
I believe the answer is:
border
'frontier' is the definition.
(I've seen this before)
'british command' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'command' becomes 'order' (order is a kind of command).
'b'+'order'='BORDER'
'having' acts as a link.
(Other definitions for border that I've seen before include "Line between countries" , "Quantity; song" , "Boundary line" , "Margin along the edge" , "Type of sheepdog" .)