Bulletin board fastener (4)
I believe the answer is:
tack
'bulletin board fastener' 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 "Small broad-headed nail" , "Nail; course of action" , "Alter course - riding gear" , "Food; nail" , "Poor food" .)