Best voice lacks edge when confined to thorax (8)

I believe the answer is:

choicest

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'best voice' is the definition.
'choicest' can be an answer for 'best' (I've seen this in another clue). I am not sure about the 'voice' bit.

'lacks edge when confined to thorax' is the wordplay.
'lacks edge when' becomes 'oic' (I am not sure about this - if you are sure you should believe this answer much more).
'confined to' means one lot of letters goes inside another.
'thorax' becomes 'chest' (synonyms).
'oic' put into 'chest' is 'CHOICEST'.

Can you help me to learn more?

(Other definitions for choicest that I've seen before include "Top" , "Best" .)

I've seen this clue in The Irish Times.

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