Pondering bumping off family in obsession (5)
I believe the answer is:
thing
'obsession' is the definition.
(thesaurus)
'pondering bumping off family' is the wordplay.
'pondering' becomes 'thinking' (I've seen this before).
'bumping off' is a deletion indicator.
'family' becomes 'kin' ('kin' can be a synonym of 'family').
'thinking' with 'kin' removed is 'THING'.
'in' is the link.
(Other definitions for thing that I've seen before include "An unnamed object or item" , "Article" , "Inanimate object" , "See 1" , "Individual unit" .)