Mad chaps beginning to talk, interrupting performance (8)
I believe the answer is:
demented
'mad' is the definition.
('demented' can be a synonym of 'mad')
'chaps beginning to talk interrupting performance' is the wordplay.
'chaps' becomes 'men' (I've seen this before).
'beginning to' suggests taking the first letters.
'interrupting' means one lot of letters goes inside another.
'performance' becomes 'deed' (I've seen this in other clues).
The first letter of 'talk' is 't'.
'men'+'t'='ment'
'ment' put within 'deed' is 'DEMENTED'.
(Other definitions for demented that I've seen before include "Crazy, insane" , "Wild, irrational" , "Nutty" , "Deranged, crazy" , "Out of one's mind" .)