Gain control of (6)
I believe the answer is:
master
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gain control of</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 master that I've seen before include "Strong chess player" , "Lord of a stream" , "get control over" , "'Jack of all trades and . . . . . . of none'" , "controlling" .)