Worried terrible old brute (8)
I believe the answer is:
troubled
'worried' is the definition.
('troubled' can be a synonym of 'worried')
'terrible old brute' is the wordplay.
'terrible' indicates an anagram.
'old'+'brute'='oldbrute'
'oldbrute' is an anagram of 'TROUBLED'.
(Other definitions for troubled that I've seen before include "Bothered, disquieted" , "Disturbed, bothered" , "Worried" , "Inconvenienced" , "Showing anxiety" .)