Release Dad’s sister who lost her head (5)

I believe the answer is:

untie

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'release' is the definition.
(eg to untie restraints)

I can't explain the remainder of the clue.

(Other definitions for untie that I've seen before include "Disconnect; release" , "Unite (anag)" , "Release bindings or shoelaces" , "Release, loosen" , "Set free" .)

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

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