Estimate the speed (4)
I believe the answer is:
rate
I believe this clue is a double definition.
'estimate' is the first definition.
(I've seen this before)
'the speed' is the second definition.
(the speed at which something happens)
(Other definitions for rate that I've seen before include "Tear up in tempo" , "Judge" , "Amount measures against another" , "See 16" , "Degree, proportion, scale" .)