A headlock (5)

I believe the answer is:

tress

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'a headlock' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

This is the entire clue.

Can you help me to learn more?

(Other definitions for tress that I've seen before include "Switch" , "Long lock or plait of hair" , "Obsolete support" , "Long piece of hair" , "Braid of hair" .)

I've seen this clue in The Globe and Mail.

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