Mark of important day for Moscow landlord (3,6)
I believe the answer is:
red letter
'mark of important' is the definition.
I can't tell whether this defines the answer.
'day for moscow landlord' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'for' means one lot of letters go next to another (I've seen this in other clues).
'moscow' becomes 're' (I am not sure about this - if you are sure you should believe this answer much more).
'landlord' becomes 'letter' (landlords let property).
'd' put after 're' is 'red'.
'red'+'letter'='RED-LETTER'
Can you help me to learn more?
(Other definitions for red letter that I've seen before include "Special" , "noteworthy" .)