Buster did become unhinged (9)
I believe the answer is:
disturbed
'unhinged' is the definition.
(I know that unhinged can be written as disturbed)
'buster did become' is the wordplay.
'become' indicates anagramming the letters.
'buster'+'did'='busterdid'
'busterdid' is an anagram of 'DISTURBED'.
(Other definitions for disturbed that I've seen before include "Changed the set arrangement or made uneasy" , "Emotionally upset -- worried" , "Troubled" , "Intruded on" , "Agitated" .)