Firmly attach small part of body (4)
I believe the answer is:
nail
'firmly attach' is the definition.
(attach with nails)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for nail that I've seen before include "Fix with a metal spike" , "Brad or tack" , "Metal pin, hit it on the head" , "thing for keeping things together" , "Brad, for example" .)