Charge at speed (4)
I believe the answer is:
rate
I believe this clue is a double definition.
'charge' is the first definition.
(rate is a kind of charge)
'speed' is the second definition.
(rate is a kind of speed)
'at' is the link.
(Other definitions for rate that I've seen before include "Assess; scold" , "Measure of frequency; merit" , "The number of times something happens in a given period" , "Assess the value of" , "Relative speed (of progress)" .)