Theory shows obsession about family (8)
I believe the answer is:
thinking
'theory' is the definition.
(thesaurus)
'obsession about family' is the wordplay.
'obsession' becomes 'thing' (I've seen this before).
'about' indicates putting letters inside.
'family' becomes 'kin' (synonyms).
'thing' going around 'kin' is 'THINKING'.
'shows' acts as a link.
(Other definitions for thinking that I've seen before include "Cogitating" , "Pondering" , "Rational" , "Fancying" , "rationale" .)