Chess player who starts with unusual energy (5)
I believe the answer is:
white
'chess player who starts' is the definition.
(the chess player with white pieces moves first)
'with unusual energy' is the wordplay.
'unusual' is an anagram indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'with' with letters rearranged gives 'whit'.
'whit'+'e'='WHITE'
(Other definitions for white that I've seen before include "Very pale; morally pure" , "Covered in snow" , "See 10" , "The colour that reflects all light" , "Pallid" .)