Waters down tea aunt brewed on upturned box (10)

I believe the answer is:

attenuates

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'waters down' is the definition.
The definition and answer can be both related to change as well as being verbs in their -s form.
Perhaps you can see a link between them that I don't see?

'tea aunt brewed on upturned box' is the wordplay.
'brewed' indicates an anagram.
'on upturned box' becomes 'tes' (I am not sure about this - if you are sure you should believe this answer much more).
'tea'+'aunt'='teaaunt'
'teaaunt' is an anagram of 'attenua'.
'attenua'+'tes'='ATTENUATES'

Can you help me to learn more?

I've seen this clue in The Independent.

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