An essential part of thieves' language (5)

I believe the answer is:

slang

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'an essential part' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'of thieves' language' is the wordplay.
'of' indicates a hidden word.
'SLANG' can be found hidden inside 'thieves language'.

Can you help me to learn more?

(Other definitions for slang that I've seen before include "Argot" , "Informal lingo" , "Patois" , "Colloquial language" , "Insult" .)

I've seen this clue in The Globe and Mail.

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