Make still or quiet (6)
I believe the answer is:
becalm
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">make still or quiet</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 becalm that I've seen before include "Make quiet or still - sounds like an order" , "Render motionless from lack of wind" , "Leave a sailing vessel unable to move for lack of wind" , "Leave (sailing ship) without wind" , "Leave (sailing ship) deprived of power" .)