Special celebration for Socialist landlord, always pursuing end of need (3,6,3)
I believe the answer is:
red letter day
'special celebration' is the definition.
(a red letter day is a special occasion)
'socialist landlord always pursuing end of need' is the wordplay.
'socialist' becomes 'red' (red is a kind of socialist).
'landlord' becomes 'letter' (landlords let property).
'always' becomes 'ay'.
'pursuing' means one lot of letters go next to another.
'end of' suggests the final letters.
The final letter of 'need' is 'd'.
'ay' put after 'd' is 'day'.
'red'+'letter'+'day'='RED-LETTER DAY'
'for' is the link.
(Other definitions for red letter day that I've seen before include "memorable time" , "Memorable event" , "Important occasion" .)