Periods set up, one's intervals (9)

I believe the answer is:

semitones

'intervals' is the definition.
(I know this)

'periods set up one's' is the wordplay.
'periods' becomes 'times' (time is a kind of period).
'set up' is a reversal indicator (in a down clue, letters go up).
'times' written backwards gives 'semit'.
'semit'+'ones'='SEMITONES'

I've seen this clue in The Telegraph.

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