A prisoner entering with a snake (8)

I believe the answer is:

anaconda

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'a snake' is the definition.
(anaconda is a kind of snake)

'a prisoner entering with' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'entering' is an insertion indicator.
'with' becomes 'nada' (I can't justify this - if you can you should give a lot more credence to this answer).
'con' inserted within 'nada' is 'naconda'.
'a'+'naconda'='ANACONDA'

Can you help me to learn more?

(Other definitions for anaconda that I've seen before include "South American killer" , "cold-blooded killer" , "1 of 21 [SNAKES]" , "The largest snake" , "Water boa" .)

I've seen this clue in The Independent.

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