The capital of Belgium commands the frontiers (7)
I believe the answer is:
borders
'frontiers' is the definition.
(I know this)
'the capital of belgium commands' is the wordplay.
'the capital of' suggests taking the first letters.
'commands' becomes 'orders' (order is a kind of command).
The initial letter of 'belgium' is 'b'.
'b'+'orders'='BORDERS'
'the' is the link.
(Other definitions for borders that I've seen before include "Boundary lines" , "Perimeters" , "Decorative edges" .)