Changing usual partner is miraculous (12)
I believe the answer is:
supernatural
'is miraculous' is the definition.
(similar in meaning)
'changing usual partner' is the wordplay.
'changing' is an anagram indicator.
'usual'+'partner'='usualpartner'
'usualpartner' anagrammed gives 'SUPERNATURAL'.
(Other definitions for supernatural that I've seen before include "Mysterious" , "Psychic" , "outside the known law?" , "unknown to science" , "Magical; occult" .)