Disciple’s letters covered in beer (7)
I believe the answer is:
apostle
'disciple's' is the definition.
(the apostles were disciples of Jesus)
'letters covered in beer' is the wordplay.
'letters' becomes 'post' (letters in the mail or post).
'covered in' means one lot of letters goes inside another.
'beer' becomes 'ale' (type of beer).
'post' placed within 'ale' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "St James, for example" , "James, perhaps" , "Leaps to find teacher of faith" , "A follower of Jesus" , "Originally, one sent to preach the gospel" .)