'Not speaking in here', says Michael Cain, the rebel! (8)
I believe the answer is:
mutineer
'the rebel' is the definition.
('mutineer' can be a synonym of 'rebel')
'not speaking in here says michael cain' is the wordplay.
I cannot really see how this works, but
'in' is within the answer.
'not speaking' could be 'mute' (I've seen this before) and 'mute' is present in the remaining letters.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mutineer that I've seen before include "Military rebel" , "One rebelling against own military authority" , "Seafarer or soldier who refuses to obey authorities" , "Participant in rebellion" , "Openly rebellious person" .)