Decide to participate (5)
'decide to participate' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for opt in that I've seen before include "Join voluntarily" , "Choose to take part" , "Choose to participate" , "Choose to be included" , "be involved voluntarily" .)
(Another definition for optin that I've seen is " Decide to take part".)