Not an outstanding form of demo? (3-2)

I believe the answer is:

sit-in

I'm a little stuck... Click here to teach me more about this clue!

'demo?' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for sit-in that I've seen before include "Occupation of a place as a protest" , "Act of civil disobedience" , "Seated protest" , "Protest demo" , "Organised passive protest" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app