Driving event undoubtedly lacks energy (5)
I believe the answer is:
rally
'driving event' is the definition.
'rally' can be an answer for 'event' (rally is a kind of event). I am not certain of the 'driving' bit.
'undoubtedly lacks energy' is the wordplay.
'undoubtedly' becomes 'really' (I've seen this before).
'lacks' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'really' with 'e' taken away is 'RALLY'.
Can you help me to learn more?
(Other definitions for rally that I've seen before include "Recuperate in gathering" , "Large gathering" , "exchange over the Net?" , "Revive - motoring contest" , "Get better; car meet" .)