Bird's speed (4)
I believe the answer is:
knot
This clue is a double definition.
'bird's' is the first definition.
(knot is a kind of bird)
'speed' is the second definition.
(thesaurus)
(Other definitions for knot that I've seen before include "Reef or granny, say" , "Eskimo people" , "A bowline or a reef" , "Entangle" , "It makes fast" .)