Unit of computer data left out of great deal (4)
I believe the answer is:
word
'unit of computer data' is the definition.
(word is a basic unit of computer data)
'left out of great deal' is the wordplay.
'left' becomes 'l' (common abbreviation).
'out of' indicates named letters should be taken away.
'great deal' becomes 'world' (as in 'that means the world').
'world' with 'l' removed is 'WORD'.
(Other definitions for word that I've seen before include "Message" , "Remark; news" , "Promise; news" , "Puzzle with solutions reading the same down as across" , "News; speech element" .)