In the main, he's right (6)
I believe the answer is:
martin
'right' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'in the main he's' is the wordplay.
I cannot quite understand how this works, but
'in' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'he' could be 'mar' and 'mar' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for martin that I've seen before include "Bird or boy's name" , "-- Chuzzlewit (Dickens)" , "Male name, bird" , "Variety of swallow" , "Antrim (anag)" .)