Engage as a participant (7)
I believe the answer is:
involve
'engage as a participant' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for involve that I've seen before include "Cause participation of (someone)" , "Implicate or be bound up with" , "Include as a part or result" , "Cause to share in" , "Entail" .)