Lady of letters has appointment with teacher (12)
I believe the answer is:
postmistress
'lady of letters' is the definition.
(woman responsible for a postal service)
'appointment with teacher' is the wordplay.
'appointment' becomes 'post' (appointment to a job).
'with' says to put letters next to each other.
'teacher' becomes 'mistress' (mistress is a kind of teacher).
'post'+'mistress'='POSTMISTRESS'
'has' is the link.
(Other definitions for postmistress that I've seen before include "Office manager" , "Local head in mail service" , "Mail manager" .)