Storage unit, a huge mouthful to say (8)
I believe the answer is:
megabyte
'storage unit' is the definition.
(I've seen this in another clue)
'a huge mouthful to say' is the wordplay.
'a huge' becomes 'mega' (mega can mean very large).
'mouthful' becomes 'bite' (a bite of food).
'to say' indicates a 'sounds like' (homophone) clue.
'bite' is a homophone of 'byte'.
'mega'+'byte'='MEGABYTE'
(Other definitions for megabyte that I've seen before include "Unit of computer memory; gate by me (anag.)" , "1,048,576 bytes" , "Mb expanded" , "Unit of information storage; get maybe (anagram)" , "Amount of computer memory - get maybe (anag)" .)