Class this month one impossible to change (3,2,5)
I believe the answer is:
set in stone
'impossible to change' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'class this month one' is the wordplay.
I cannot quite understand how this works, but
'class' could be 'set' (I've seen this in other clues) and 'set' is located in the answer.
'month' could be 'inst' (I've seen this before) and 'inst' is located in the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
The remaining letters 'ne' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for set in stone that I've seen before include "Fixed and unalterable" , "As inflexible" , "as Excalibur?" .)