Landlord's post (7)
I believe the answer is:
letters
'landlord's post' is the definition.
'letters' can be an answer for 'post' (thesaurus). I am unsure of the 'landlord's' bit.
'landlord's post' is the wordplay.
'landlord' becomes 'letter' (landlords let property).
'post' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'letter'+'s'='LETTERS'
Can you help me to learn more?
(Other definitions for letters that I've seen before include "providing digs?" , "They may indicate a degree" , "academic credentials" , "They are bound to look after tenants" , "Characters" .)