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 justify this - if you can you should give a lot more credence to this answer).
'some' says to take the initial letters.
'back' says the letters should be written backwards.
The initial letters of 'semitones echo' is 'se'.
'se' backwards is 'es'.
'tim'+'es'='TIMES'

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(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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