It may be tied at speed (4)
I believe the answer is:
knot
This clue is a double definition.
'it may be tied' is the first definition.
(I've seen this before)
'speed' is the second definition.
(knot is a nautical measure of speed)
'at' acts as a link.
(Other definitions for knot that I've seen before include "Granny or reef, perhaps?" , "Measure of speed at sea" , "See 2" , "Tied fastening" , "Could be a granny or a sheepshank" .)