They're not the key parts of firearms (5)
I believe the answer is:
locks
'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" .)