Opinionated scoundrel has mum twitching (8)
I believe the answer is:
dogmatic
'opinionated' is the definition.
(thesaurus)
'scoundrel has mum twitching' is the wordplay.
'scoundrel' becomes 'dog' (dog is a kind of scoundrel).
'has' means one lot of letters go next to another.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'twitching' becomes 'tic' (tic is a kind of twitching).
'dog'+'ma'+'tic'='DOGMATIC'
(Other definitions for dogmatic that I've seen before include "Giving opinions in arrogant know-all way" , "Assertive, overbearing" , "Asserting things as incontrovertibly true" , "Putting forward opinions in authoritative or intolerant way" , "Asserting positively" .)