Initially having abandoned speed, takes excessive speed (5)
I believe the answer is:
haste
'speed' is the definition.
(haste is a kind of speed)
'initially having abandoned speed takes excessive' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'having abandoned speed takes excessive' is 'HASTE'.
(Other definitions for haste that I've seen before include "Excessive speed or urgency" , "The more of it, the less speed sometimes" , "More of this can slow you up!" , "Urgency of action" , "Undue speed" .)