Letters of appointment (4)
I believe the answer is:
post
I believe this clue is a double definition.
'letters' is the first definition.
(thesaurus)
'appointment' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for post that I've seen before include "Stake, job or mail" , "Letters and parcels; job" , "delivery" , "Upright length of wood or metal a permanent military establishment" , "(Holiday) communication" .)