Moderates, as is one's wont, changing key in (5,4)

'moderates' is the definition.
('tone down' can be a synonym of 'moderate')

'is one's wont changing key in' is the wordplay.
I cannot quite see how this works, but
'one's' could be 'ones' and 'ones' is found within the answer.
'key' could be 'd' (I've seen this in other clues) and 'd' is located in the answer.
an anagram of 'wont' is 'town' which is within the remaining letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).

'as' acts as a link.

Can you help me to learn more?