Bulletin board pin (4)
I believe the answer is:
tack
'bulletin board pin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tack that I've seen before include "Stickiness" , "Nail; course of action" , "Nail; harness" , "Short, flat-headed nail" , "Fix with pins, shoddy material" .)