Increase twofold (6)
I believe the answer is:
double
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">increase twofold</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 double that I've seen before include "Designed for two users" , "Increase twofold" , "Offence of leaving one's car where it blocks another" , "Bed for two people" , "Twice, the size say" .)