He is, in the main, right (6)
I believe the answer is:
martin
'he' is the definition.
'in the main right' is the wordplay.
'in the' indicates putting letters inside.
'right' becomes 'rt'.
'main' enclosing 'rt' is 'MARTIN'.
'is' is the link.
(Other definitions for martin that I've seen before include "Creature that flies" , "Perhaps Dean" , "Swallow-like bird" , "25 across [CRANE]" , "Antrim (anag)" .)