Fastening – sandpiper (4)
I believe the answer is:
knot
I believe this clue is a double definition.
'fastening' is the first definition.
(I know that knot is a type of fastening)
'sandpiper' is the second definition.
(knot is a kind of sandpiper)
(Other definitions for knot that I've seen before include "measure of pace" , "Fastener - bird - unit of speed at sea" , "bond, perhaps?" , "Air mile" , "Ring < sandpiper" .)