Not initially full, not initially empty (4)
I believe the answer is:
null
'empty' is the definition.
(thesaurus)
'not initially full not initially' is the wordplay.
'initially' indicates taking the first letters.
'not initially' means to remove the first letter.
The first letter of 'not' is 'n'.
'full' with its first letter taken off is 'ull'.
'n'+'ull'='NULL'
(Other definitions for null that I've seen before include "Non-existent and void" , "Not valid" , "... and void, lacking any legal or binding force" , "Invalid - not binding" , "Lacking any legally binding force" .)