Very large shop (10)
'very large shop' is the definition.
(I've seen this before)
This is all the clue.
(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" .)