Increase threatening sound, scaring off Liberal (4)

I believe the answer is:

grow

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

'increase' is the definition.
(I know that grow is a more specific form of increase)

I don't understand how the remainder of the clue works.

(Other definitions for grow that I've seen before include "Expand" , "Swell" , "Mature" , "Develop; increase" , "Become bigger" .)

I've seen this clue in The Independent.

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