It makes landlord chief write under this (10)
I believe the answer is:
letterhead
'it makes landlord' is the definition.
The definition suggests a singular noun which matches the answer.
'chief write under this' is the wordplay.
I cannot really see how this works, but
'chief' could be 'head' (synonyms) and 'head' is found in the answer.
'write' could be 'letter' (lettering is a kind of writing) and 'letter' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for letterhead that I've seen before include "Where name and address are shown" , "Heading on stationery" , "Address, name etc printed at top of notepaper" , "Preprinted name, address etc at top of notepaper" , "Printed address on stationery" .)