Make amends for a firmness of body (5)

I believe the answer is:

atone

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'make amends for' is the definition.
(I've seen this before)

'a firmness of body' is the wordplay.
'firmness of body' becomes 'tone' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'tone'='ATONE'

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(Other definitions for atone that I've seen before include "In agreement - sounds singular" , "Turn away from sin and do penance" , "Make reparation for a misdeed" , "Make amends, being at one" , "Make amends (for something)" .)

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

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