Heading off some time around dusk, not fixed (5)

I believe the answer is:

unset

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'dusk not fixed' is the definition.
I can't judge whether this definition defines the answer.

'heading off some time' is the wordplay.
'heading off' means to remove the first letter.
'some time' becomes 'sunset' (sunset is a kind of time).
'sunset' with its first letter taken off is 'UNSET'.

'around' acts as a link.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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