Add compound interest to begin with or default badly (10)

I believe the answer is:

fluoridate

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'add compound' is the definition.
I can't judge whether this definition defines the answer.

'interest to begin with or default badly' is the wordplay.
'to begin with' indicates taking the first letters.
'badly' is an anagram indicator (letters in the wrong order).
The initial letter of 'interest' is 'i'.
'i'+'or'+'default'='iordefault'
'iordefault' anagrammed gives 'FLUORIDATE'.

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