Handles decrease in change to straight thinking (5,10)

I believe the answer is:

clear headedness

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

'straight thinking' is the definition.
The definition suggests a singular noun which matches the answer.

'handles decrease in change' is the wordplay.
'handles decrease' anagrams to 'CLEAR HEADEDNESS'.
Though, I am unsure how this is indicated.

'to' acts as a link.

Can you help me to learn more?

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

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