Walk at speed (4)
I believe the answer is:
pace
This clue is a double definition.
'walk' is the first definition.
(I know that walk is a more specific form of the action pace)
'speed' is the second definition.
(pace is a kind of speed)
'at' acts as a link.
(Other definitions for pace that I've seen before include "Gait" , "Part of step" , "Rate of progress" , "Clip" , "a certain distance" .)