Letters set down on ground (9)
I believe the answer is:
landlords
'letters' is the definition.
(people who let property)
'set down on ground' is the wordplay.
'set down' becomes 'land' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'ground' becomes 'lords' (I've seen this before).
'land'+'lords'='LANDLORDS'
(Other definitions for landlords that I've seen before include "letters" , "maybe posted locally?" , "People in pubs" , "They lease out properties" .)