Conditions worsen in fashion dominion (9)
I believe the answer is:
ownership
'dominion' is the definition.
(thesaurus)
'conditions worsen in fashion' is the wordplay.
'conditions' indicates anagramming the letters (letters must be in a new condition).
'in fashion' becomes 'hip' (I've seen this in another clue).
'worsen' anagrammed gives 'owners'.
'owners'+'hip'='OWNERSHIP'
(Other definitions for ownership that I've seen before include "Legal title" , "Proprietorship, possession" , "The right of possessing something" , "Proprietary right of possession" .)