Large prefix (4)
I believe the answer is:
mega
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">large prefix</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for mega that I've seen before include "Smashing" , "ace" , "extremely good" , "As brilliant" , "Informally great or large" .)