To a great extent (7)
I believe the answer is:
largely
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">to a great extent</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 largely that I've seen before include "to a considerable extent" , "Generally" , "In the main" , "Mainly" , "Mostly found in gallery" .)