Master knob (4)
I believe the answer is:
boss
I believe this is a double definition.
'master' is the first definition.
(thesaurus)
'knob' is the second definition.
(thesaurus)
(Other definitions for boss that I've seen before include "Head honcho" , "Person in charge - raised knob" , "Governor" , "Domineer" , "Behave overbearingly" .)