Crossword Genius

They're not the key parts of firearms (5)

Ross

I believe the answer is:

locks

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

'firearms' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Perhaps you can see a link between them that I can't see?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for locks that I've seen before include "Door fasteners" , "Secures; person's hair" , "Tresses" , "raising devices for sailors" , "Canal features" .)

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