An entrant's hips are unstable. That's why she's fidgeting (4,2,3,5)

I believe the answer is:

ants in her pants

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'why she's fidgeting' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'an entrant's hips are unstable' is the wordplay.
'are unstable' indicates an anagram.
'entrants'+'hips'='entrantships'
'entrantships' with letters rearranged gives 'tsinherpants'.
'an'+'tsinherpants'='ANTS IN HER PANTS'

'that's' is the link.

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I've seen this clue in the Sydney Morning Herald.

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