Used to make a lock fast, or to pick it (7)

I believe the answer is:

hairpin

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'used to make a lock' is the definition.
I know that 'lock' relates to this answer.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for hairpin that I've seen before include "Curving sharply" , "stuck in a lock?" , "Very sharp bend in mountain road" , "Nasty road bend or aid to coiffure" , "description of the bends?" .)

I've seen this clue in The Irish Independent.

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