Crossword Genius

Be anxious when losing opener for lock (5)

Ross

I believe the answer is:

tress

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

This could be a double definition.

'be anxious when losing opener' is the first definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'for lock' is the second definition.
(both can mean a piece of hair)

Can you help me to learn more?

(Other definitions for tress that I've seen before include "Switch" , "Rests (anag.)" , "Long piece of hair" , "Braid of hair" , "Curl - lock" .)

I've seen this clue in The Times.
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