Excalibur perhaps was, and is not going to change (3,2,5)
I believe the answer is:
set in stone
'not going to change' is the definition.
'excalibur perhaps was and' is the wordplay.
I cannot really see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
'was' could be 'is' (being is a kind of being) and 'is' is located in the leftover letters.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for set in stone that I've seen before include "As inflexible" , "Fixed and unalterable" , "as Excalibur?" .)