Observe the change of tone (4)

I believe the answer is:

note

'observe' is the definition.
(I know that observe can be written as note)

'change of tone' is the wordplay.
'change of' indicates an anagram (the letter order is changed).
'tone' is an anagram of 'NOTE'.

'the' is the link.

(Other definitions for note that I've seen before include "A, B, C, D, F or G" , "mention" , "Eg, fiver; annotation" , "Traitor" , "A flat, for example" .)

I've seen this clue in the Evening Standard.

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