Old nag he whipped, at breakneck speed (8)
I believe the answer is:
headlong
'at breakneck speed' is the definition.
(I've seen this before)
'old nag he whipped' is the wordplay.
'whipped' indicates anagramming the letters.
'old'+'nag'+'he'='oldnaghe'
'oldnaghe' anagrammed gives 'HEADLONG'.
(Other definitions for headlong that I've seen before include "Hasty and rash, falling or plunging" , "Impetuous" , "Precipitate (flight)" , "At breakneck speed" , "With reckless haste" .)