Bird's measure of speed at sea (4)
I believe the answer is:
knot
This clue is a triple definition!
'bird's' is the first definition.
(knot is a kind of bird)
'measure' is the second definition.
(knot is a kind of measure)
'of' acts as a link.
'speed at sea' is the third definition.
(measure of speed once used on ships)
(Other definitions for knot that I've seen before include "Hard irregularity in timber - interlacing of cord" , "Bird; tie securely" , "Shore bird, tangle" , "Bend - one nautical mile per hour" , "Flier" .)