Estimate speed (4)
I believe the answer is:
rate
This clue is a double definition.
'estimate' is the first definition.
(both can mean to judge)
'speed' is the second definition.
(both can mean pace)
(Other definitions for rate that I've seen before include "The speed of movement" , "See 16" , "Merit; speed" , "Assess; be worthy of" , "Admire" .)