Start a movement that will make big news (5,1,4)

I believe the answer is:

cause a stir

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

'start a movement' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'will make big news' is the wordplay.
'will' means one lot of letters go next to another (I've seen this in other clues).
'make' becomes 'cause' (causing is a kind of making).
'big news' becomes 'astir' (I can't explain this - if you can you should give a lot more credence to this answer).
'cause' put next to 'astir' is 'CAUSE A STIR'.

'that' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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.