Most embarrassed to be worst (7)

I believe the answer is:

poorest

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'worst' is the definition.
(similar in meaning)

'most embarrassed to be' is the wordplay.
'most embarrassed' becomes 'poo' (I am not sure about this - if you are sure you should believe this answer much more).
'to be' becomes 'rest' (resting is a kind of being).
'poo'+'rest'='POOREST'

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(Another definition for poorest that I've seen is " Of the lowest quality".)

I've seen this clue in the Evening Standard.

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