Very large shop (9)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">very large shop</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 megastore that I've seen before include "Very large shop" , "giant in business" , "large outlet" , "Massive outlet" .)
(Other definitions for superstore that I've seen before include "Retailer" , "Large out-of-town shop" .)