Unit of computer data (8)
(Other definitions for megabyte that I've seen before include "amount of information" , "Unit of information storage; get maybe (anagram)" , "Mb expanded" , "Unit of computer memory; gate by me (anag.)" , "Hundreds of thousands of words" .)
(Other definitions for byte that I've seen before include "Unit of (usually eight) bits" , "Set of eight bits" , "Unit of data storage in computer" , "Computing unit" , "Computer's unit of stored memory" .)