Free relative forgoes introduction (5)

I believe the answer is:

untie

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'free' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

(Other definitions for untie that I've seen before include "Take off fastenings" , "Set free" , "Undo, unfasten" , "Undo, unlace" , "Free from binds" .)

I've seen this clue in The Times.

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