Opening move (6)
I believe the answer is:
gambit
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">opening move</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 gambit that I've seen before include "tricky move" , "play involving sacrifice" , "Opening ploy in chess" , "Opening action" , "Tactic intended to gain an advantage" .)