Parties if backseat's unhinged (7)

I believe the answer is:

fiestas

I'm a little stuck... Click here to teach me more about this clue!

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

I don't understand the remainder of the clue.

(Other definitions for fiestas that I've seen before include "It's safe to have public celebrations in Spain" , "holidays ..." .)

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

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