In charge of energy reserve (3)
I believe the answer is:
ice
'reserve' is the definition.
(both can mean aloofness)
'in charge of energy' is the wordplay.
'in charge of' becomes 'ic'.
'energy' becomes 'e' (physics symbol as in E=mc2).
'ic'+'e'='ICE'
(Other definitions for ice that I've seen before include "Diamonds (slang)" , "Cool down" , "Some number" , "Diamonds (informal)" , "Freeze" .)