American monk with a halo that’s slipping (9)
I believe the answer is:
Here is my best explanation:
'american' is the definition.
'monk with a halo that's slipping' is the subsidiary indication.
'with' says to put letters next to each other.
'that's slipping' is an anagram indicator.
'monk' put after 'ahalo' is 'ahalomonk'.
'ahalomonk' anagrammed gives 'oklahoman'.
(Other definitions for oklahoman that I've seen before include "from Tulsa?", "American" and "US resident".)