Usual partner turning abnormal (12)
I believe the answer is:
supernatural
'abnormal' is the definition.
(thesaurus)
'usual partner turning' is the wordplay.
'turning' indicates anagramming the letters.
'usual'+'partner'='usualpartner'
'usualpartner' anagrammed gives 'SUPERNATURAL'.
(Other definitions for supernatural that I've seen before include "Mysterious" , "Not of this world" , "unknown to science" , "Magic" , "Magical; occult" .)