Be calm when pay reduced .. (6,4)

I believe the answer is:

settle down

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'be calm' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Perhaps there's a link between them I don't understand?

'pay reduced' is the wordplay.
'pay' becomes 'settle' (as in settling a bill).
'reduced' becomes 'down'.
'settle'+'down'='SETTLE DOWN'

'when' acts as a link.

Can you help me to learn more?

(Other definitions for settle down that I've seen before include "put down roots" , "Become quiet and orderly" .)

I've seen this clue in The Irish Times.

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