Landlords in mail? (7)
I believe the answer is:
letters
This clue is a double definition.
'landlords' is the first definition.
(people who let property)
'mail?' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for letters that I've seen before include "Written notes" , "Items of mail" , "A, B, C, D etc" , "Characters" , "landlords?" .)