Anima run riot in obsession (5)
I believe the answer is:
mania
'obsession' is the definition.
(I've seen this before)
'anima run riot' is the wordplay.
'run riot' indicates anagramming the letters (I've seen this in other clues).
'anima' anagrammed gives 'MANIA'.
'in' is the link.
(Other definitions for mania that I've seen before include "Mental disorder that results in excessive or violent responses" , "wildness" , "Excessive enthusiasm" , "Craze; overexcitement" , "Over-enthusiasm" .)