Lookout soldier (6)
I believe the answer is:
sentry
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lookout soldier</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for sentry that I've seen before include "Person posted to keep watch for some anticipated event" , "Sentinel, watch" , "Soldier on guard" , "Watchperson" , "Military lookout" .)