Disintegrating cotton on feathers (10)
I believe the answer is:
tumbledown
'disintegrating' is the definition.
(thesaurus)
'cotton on feathers' is the wordplay.
'cotton on' becomes 'tumble' ('tumble' can be a synonym of 'cotton on').
'feathers' becomes 'down' (down is a kind of feathers).
'tumble'+'down'='TUMBLEDOWN'
(Other definitions for tumbledown that I've seen before include "neglected" , "threatening to collapse" , "Ramshackle" , "Derelict" , "in a dilapidated state" .)