Demon is in to change size (9)
I believe the answer is:
dimension
'size' is the definition.
(I know that size can be written as dimension)
'demon is in to change' is the wordplay.
'to change' indicates an anagram.
'demon'+'is'+'in'='demonisin'
'demonisin' is an anagram of 'DIMENSION'.
(Other definitions for dimension that I've seen before include "Length or breadth, say" , "measure" , "Proportion" , "size" .)