Graceful runner with energy — and light (5)
I believe the answer is:
eland
'graceful runner' is the definition.
(type of antelope)
'energy and light' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mca).
'and' means one lot of letters go next to another.
'light' becomes 'land' (light can mean to settle or land).
'e'+'land'='ELAND'
'with' acts as a link.
(Other definitions for eland that I've seen before include "wild creature" , "game animal" , "Laden (anag.); antelope" , "Wild beast" , "Led an (anag) -- antelope" .)