Core group forsakes degenerate waster - most rational (6)

I believe the answer is:

sanest

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'most rational' is the definition.
('sane' can be similar in meaning to 'rational')

I can't explain the rest of the clue.

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(Other definitions for sanest that I've seen before include "Most rational and logical" , "Most reasoned and mentally sound" , "Most rational and reasonable" , "Mentally most healthy" , "Most mentally well" .)

I've seen this clue in The Irish Times.

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