Literally resign as number one (6-2-7)
I believe the answer is:
second-in-command
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'literally resign as number' is the wordplay.
I cannot quite see how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is present in the answer.
'number' could be 'm' (M is an example) and 'm' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for second-in-command that I've seen before include "Deputy" , "Officer immediately below chief officer" .)