Undertake to get into gear? (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 "Pay to serve" , "Employ or captivate" , "Secure" , "Employ, hire" , "Employ as a worker" .)