Occasions when some semitones echo back (5)

I believe the answer is:

times

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'occasions' is the definition.
(I've seen this before)

'when some semitones echo back' is the wordplay.
'when' becomes 'tim' (I can't explain this - if you can you should believe this answer much more).
'some' suggests taking the first letters.
'back' is a reversal indicator.
The first letters of 'semitones echo' is 'se'.
'se' back-to-front is 'es'.
'tim'+'es'='TIMES'

Can you help me to learn more?

(Other definitions for times that I've seen before include "Periods, ages" , "Paper" , "See 1" , "Big Apple's theatre district" , "Paces" .)

I've seen this clue in the Evening Standard.

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