Travel using free energy (4)
I believe the answer is:
ride
'travel' is the definition.
(riding is a kind of travelling)
'free energy' is the wordplay.
'free' becomes 'rid' ('rid' can be a synonym of 'free'**).
'energy' becomes 'e' (physics symbol as in E=mci).
'rid'+'e'='RIDE'
'using' is the link.
(Other definitions for ride that I've seen before include "Go by bicycle" , "Eg, roundabout or roller-coaster, path for horses" , "Jaunt" , "Travel on the back of animal" , "Journey as passenger" .)