Letter from apostle's so righteous (6)
I believe the answer is:
lessor
'letter' is the definition.
(I've seen this before)
'from apostle's so righteous' is the wordplay.
'from' indicates the answer is hidden within the clue.
'LESSOR' is hidden within 'apostles so righteous'.
(Other definitions for lessor that I've seen before include "Property owner perhaps" , "One lets" , "Landlord" , "The granter of a lease" , "One granting a lease" .)