Injury payment certain to affect equanimity (9)

I believe the answer is:

composure

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'affect equanimity' is the definition.
'composure' can be an answer for 'equanimity' (synonyms). I am unsure of the 'affect' bit.

'injury payment certain' is the wordplay.
'injury payment' becomes 'compo' (I am not sure about this - if you are sure you should believe this answer much more).
'certain' becomes 'sure' (synonyms).
'compo'+'sure'='COMPOSURE'

'to' is the link.

Can you help me to learn more?

(Other definitions for composure that I've seen before include "State of calmness" , "Calm self-control" , "Calm state" , "Coolness" , "Calmness and self-control" .)

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

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