Principal officer was given important assignment but it wasn't mentioned, perhaps deliberately (8)
I believe the answer is:
omission
'principal officer' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'given important assignment but it wasn't mentioned perhaps deliberately' is the wordplay.
I cannot quite see how this works, but
'assignment' could be 'mission' (mission is a kind of assignment) and 'mission' is found in the answer.
A single letter 'o' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'was' acts as a link.
Can you help me to learn more?
(Other definitions for omission that I've seen before include "Something left out or not done" , "Act of leaving out" , "Oversight, piece of work neglected" , "cut?" , "type of sin" .)