Quite an occasion when the extreme left write? (3-6,3)
I believe the answer is:
red-letter day
'quite an occasion when the extreme' is the definition.
I can't judge whether this definition defines the answer.
'left write?' is the wordplay.
I cannot quite see how this works, but
'left' could be 'red' (red can informally mean communist) and 'red' is found within the answer.
'write?' could be 'letter' (lettering is a kind of writing) and 'letter' is present in the answer.
The remaining letters 'day' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for red-letter day that I've seen before include "Memorably important occasion" , "Noteworthy time" , "Memorable occasion" , "Momentous date" , "Special date in the calendar" .)