Leaving out nothing, given important task (8)
I believe the answer is:
omission
'leaving out' is the definition.
(I've seen this before)
'nothing given important task' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'given' is a charade indicator (letters next to each other).
'important task' becomes 'mission' (I've seen this before. I am not sure about the 'important' bit.).
'o'+'mission'='OMISSION'
Can you help me to learn more?
(Other definitions for omission that I've seen before include "Something left out or neglected" , "Failure to include" , "type of sin" , "Oversight, piece of work neglected" , "task not done" .)