Promise to take part in (6)
I believe the answer is:
engage
I believe this clue is a double definition.
'promise' is the first definition.
(I've seen this before)
'take part in' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for engage that I've seen before include "Join" , "Hold" , "Employ or get involved in battle" , "Become bound; attract" , "Employ or captivate" .)