Judgment of relations enveloped in obsession (8)
I believe the answer is:
thinking
'judgment' is the definition.
(thesaurus)
'relations enveloped in obsession' is the wordplay.
'relations' becomes 'kin' (I've seen this before).
'enveloped in' is an insertion indicator.
'obsession' becomes 'thing' (I've seen this before).
'kin' put into 'thing' is 'THINKING'.
'of' acts as a link.
(Other definitions for thinking that I've seen before include "Mental process" , "Fancying" , "Take" , "rationale" , "Considering in the mind" .)