Digits therefore must include megabytes (6)
I believe the answer is:
thumbs
'digits' is the definition.
(I know this)
'therefore must include megabytes' is the wordplay.
'therefore' becomes 'thus' (synonyms).
'must include' indicates putting letters inside.
'megabytes' becomes 'mb' (abbreviation).
'thus' enclosing 'mb' is 'THUMBS'.
(Other definitions for thumbs that I've seen before include "Digits; requests (lift)" , "To be all these is to be clumsy" , "reads intensively" , "Hitches a lift" , "Short thick digits" .)