Assess velocity (4)
I believe the answer is:
rate
I believe this clue is a double definition.
'assess' is the first definition.
(rating is a kind of assessing)
'velocity' is the second definition.
(rate is a kind of velocity)
(Other definitions for rate that I've seen before include "Think highly of; criticise severely" , "Cost" , "Fixed price or relative speed" , "Deserve; scold" , "The price paid" .)