Secretary's boss is a crafty type responsible for keeping the people under the thumb as a rule (12)
I believe the answer is:
dictatorship
'rule' is the definition.
(I have seen 'Absolute rule ' mean 'dictatorship' so perhaps 'rule' could also mean 'dictatorship')
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for dictatorship that I've seen before include "Absolute form of government" , "Totalitarianism" , "Absolute rule" , "Form of rule" , "Totalitarian regime" .)