Go by bike using free energy (4)
I believe the answer is:
ride
'go by bike' is the definition.
(I've seen this in another clue)
'free energy' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'energy' becomes 'e' (physics symbol as in E=mci).
'rid'+'e'='RIDE'
'using' acts as a link.
(Other definitions for ride that I've seen before include "Fairground attraction; go on horseback" , "Jaunt" , "Travel in or be carried on" , "Travel upon" , "A dire sort of journey" .)