Brad or tack, eg (4)
I believe the answer is:
nail
I believe this is a double definition.
'brad' is the first definition.
(brad is a nail with rectangular cross-section)
'tack eg' is the second definition.
(I know that tack is a type of nail)
(Other definitions for nail that I've seen before include "Hit this on the head to be bang-on" , "A small steel wood pin" , "Measure of cloth" , "Finger end; fasten" , "Spike, tack" .)