Speed of parking expert (4)
I believe the answer is:
pace
'speed' is the definition.
(pace is a kind of speed)
'parking expert' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'expert' becomes 'ace' (ace can informally mean very good).
'p'+'ace'='PACE'
'of' is the link.
(Other definitions for pace that I've seen before include "Speed of walking" , "Speed of a single step?" , "politely expressing disagreement" , "Clip" , "Tempo, rate of moving" .)