Big prefix (4)
I believe the answer is:
mega
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">big prefix</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mega that I've seen before include "Informally great or large" , "Big" , "wicked" , "Very large (colloq.)" , "Great American" .)