Decide to participate (3,2)

I think it could be either:

Which length description are you looking for?:

3,2

opt in

'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" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.