Promise to participate (6)
I believe the answer is:
engage
This clue is a double definition.
'promise' is the first definition.
(I know that engage is a more specific form of the action the vow)
'participate' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for engage that I've seen before include "Hire (a worker)" , "Employ or captivate" , "Take on - reserve - promise" , "Take on (eg for work, in battle)" , "Thief (rhyming slang)" .)