Not the way a self-possessed correspondent signs himself? (5)
I believe the answer is:
yours
'a self-possessed correspondent signs himself?' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'not the way' is the wordplay.
I cannot really see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'way' could be 's' (Compass direction - South) and 's' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for yours that I've seen before include "Belonging to you" , "Family, perhaps" , "Word found before "sincerely" or "faithfully" in a letter" , "Letter sign-off" , "ie Not mine" .)