Promise to participate (6)
I believe the answer is:
engage
I believe this is a double definition.
'promise' is the first definition.
(I know that engage is a more specific form of the action vow)
'participate' is the second definition.
(I've seen this in another clue)
'to' is the 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)" .)