Observe roles being reversed, that fixes the time (10)

I believe the answer is:

watchstrap

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'that fixes the time' is the definition.
I can't tell whether this definition defines the answer.

'observe roles being reversed' is the wordplay.
'observe' becomes 'watch' (synonyms).
'roles' becomes 'parts' ('part' can be a synonym of 'role').
'being reversed' says the letters should be written backwards.
'parts' reversed gives 'strap'.
'watch'+'strap'='WATCHSTRAP'

Can you help me to learn more?

(Another definition for watchstrap that I've seen is " Band with time".)

I've seen this clue in The Daily Express.

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