Soldier on watch (6)

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