Leaving out nothing on assignment (8)
I believe the answer is:
omission
'leaving out' is the definition.
(thesaurus)
'nothing on assignment' is the wordplay.
'nothing on' becomes 'o' (I've seen this in other clues).
'assignment' becomes 'mission' (mission is a kind of assignment).
'o'+'mission'='OMISSION'
(Other definitions for omission that I've seen before include "Something lacking" , "Something left out or neglected" , "Act of leaving out" , "lapse" , "Deficiency" .)