Guard sent to a breached line (8)
I believe the answer is:
sentinel
'guard' is the definition.
(sentinel is a kind of guard)
'sent to a breached line' is the wordplay.
'to a breached' indicates anagramming the letters.
'line' anagrammed gives 'inel'.
'sent'+'inel'='SENTINEL'
(Other definitions for sentinel that I've seen before include "Watching guard" , "Guard whose job is to stand and keep watch" , "Soldier keeping watch" , "One watches" , "Watcher on guard" .)