Be dismissed, finally paying their debt off (3,3,4)

I believe the answer is:

get the bird

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'be dismissed' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'finally paying their debt off' is the wordplay.
'finally' suggests the final letters.
'off' is an anagram indicator.
The final letter of 'paying' is 'g'.
'their'+'debt'='theirdebt'
'theirdebt' is an anagram of 'etthebird'.
'g'+'etthebird'='GET THE BIRD'

Can you help me to learn more?

(Other definitions for get the bird that I've seen before include "successful effort on Glorious Twelfth?" , "Receive expressions of disapproval" , "Retriever's job" , "Be hissed and booed on stage" , "Be hissed when performing" .)

I've seen this clue in The Independent.

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