Not all are 'sent' men, terrible bitterness being shown (10)

I believe the answer is:

resentment

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'bitterness being shown' is the definition.
I can't tell whether this definition defines the answer.

'not all are sent men terrible' is the wordplay.
'not all' says the answer is hidden in the clue (only some of the letters).
'RESENTMENT' is hidden within 'are sent men terrible'.

Can you help me to learn more?

(Other definitions for resentment that I've seen before include "Annoyance" , "Hard feelings" , "Ill will" , "Bitterness, indignation" , "Umbrage" .)

I've seen this clue in The Guardian.

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