Thief’s developed obsession (6)
I believe the answer is:
fetish
'obsession' is the definition.
(thesaurus)
'thief's developed' is the wordplay.
'developed' indicates anagramming the letters.
'thiefs' anagrammed gives 'FETISH'.
(Other definitions for fetish that I've seen before include "An abnormal object of sexual desire" , "Object worshipped as having a spirit" , "Object worshipped by primitive people or fixation (6)" , "thing?" , "Object worshipped for its supposed power" .)