Son suffering quietly being purged with new energy, becoming cheerful (8)

I believe the answer is:

sanguine

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'cheerful' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for sanguine that I've seen before include "Cheerfully optimistic" , "Bloody - cheerful" , "Optimistic, confident" , "Confident < , or > ruddy" , "Hopeful or optimistic" .)

I've seen this clue in The Telegraph.

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