Guard ordered to breached line (8)
I believe the answer is:
sentinel
'guard' is the definition.
(sentinel is a kind of guard)
'ordered to breached line' is the wordplay.
'ordered' becomes 'sent'.
'to breached' becomes 'in' (I've seen this in other clues).
'line' becomes 'el' (el is a kind of line).
'sent'+'in'+'el'='SENTINEL'
(Other definitions for sentinel that I've seen before include "One watches" , "Look-out, military guard" , "Guard who stands and keeps watch" , "Watchman" , "Soldier keeping watch" .)