Correspondence showing restraint by landlord (5,6)
I believe the answer is:
chain letter
'correspondence' is the definition.
(I have seen 'Sequential correspondence ' mean 'chain letter' so perhaps 'correspondence' could also mean 'chain letter')
'restraint by landlord' is the wordplay.
'restraint' becomes 'chain' (chain is a kind of restraint).
'by' means one lot of letters go next to another.
'landlord' becomes 'letter' (landlords let property).
'chain'+'letter'='CHAIN-LETTER'
'showing' acts as a link.
(Other definitions for chain letter that I've seen before include "Fraudulent scheme" , "Money-making scheme" , "Cascading correspondence" , "Fast-spreading communication" , "The clarinet (anag.)" .)