Drove off before one had exerted oneself unduly (8)
I believe the answer is:
overdone
'exerted oneself unduly' is the definition.
I can't judge whether this defines the answer.
'drove off before one' is the wordplay.
'off' indicates anagramming the letters.
'before' says to put letters next to each other.
'drove' with letters rearranged gives 'overd'.
'overd'+'one'='OVERDONE'
'had' is the link.
Can you help me to learn more?
(Other definitions for overdone that I've seen before include "Cooked too much or carried too far" , "Exaggerated; cooked too much" , "Hammed up" , "Cooked too long" , "really a bit much!" .)