Talk loudly of the decimal system (7)

I believe the answer is:

declaim

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'talk' is the definition.
(thesaurus)

'loudly of the decimal system' is the wordplay.
'decimal' anagrams to 'DECLAIM'.
Though, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for declaim that I've seen before include "Harangue" , "Speak dramatically" , "rant" , "attack" , "lecture" .)

I've seen this clue in the Evening Standard.

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