Grimmest earl arrested by Tudors in revolt (7)
I believe the answer is:
dourest
'grimmest' is the definition.
('dour' can be a synonym of 'grim')
'earl arrested by tudors in revolt' is the wordplay.
'earl' becomes 'e'.
'arrested by' means one lot of letters goes inside another (I've seen this in other clues).
'in revolt' indicates an anagram.
'tudors' anagrammed gives 'dourst'.
'e' inserted inside 'dourst' is 'DOUREST'.
(Other definitions for dourest that I've seen before include "Most forbidding" , "Grimmest" .)