Undertake to be the boss (6)
I believe the answer is:
engage
'undertake' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for engage that I've seen before include "Occupy (interest)" , "Recruit" , "Morally committed to" , "Participate" , "having to start a battle" .)