Choose to participate (5)
'choose 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 "be involved voluntarily" , "Choose to be included" , "Join voluntarily" , "Choose to participate" , "Choose to take part" .)
(Another definition for optin that I've seen is " Decide to take part".)