Start of our undertaking? What's been left out? (8)
I believe the answer is:
omission
'what's been left out?' is the definition.
I can't judge whether this defines the answer.
'start of our undertaking?' is the wordplay.
'start of' says to take the initial letters.
'undertaking?' becomes 'mission' (I've seen this in other clues).
The initial letter of 'our' is 'o'.
'o'+'mission'='OMISSION'
Can you help me to learn more?
(Other definitions for omission that I've seen before include "Mistake resulting from neglect" , "Failure to include" , "task not done" , "A thing left out" , "type of sin" .)