Have a high opinion of speed (4)
I believe the answer is:
rate
'speed' is the definition.
(the speed at which something happens)
'have a high opinion' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
The remaining letters 'rte' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for rate that I've seen before include "Think highly of; criticise severely" , "Merit; speed" , "Strongly criticise" , "Adjudge" , "See 11" .)