One die-hard extremist in town became less strident (5,4)

I believe the answer is:

toned down

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

'one' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for toned down that I've seen before include "Less harsh" , "Less 27a [EXTREME]" , "Moderated; softened" , "Modest" .)

I've seen this clue in the Sydney Morning Herald.

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