Reserved object with energy (6)
I believe the answer is:
demure
'reserved' is the definition.
(I've seen this in another clue)
'object with energy' is the wordplay.
'object' becomes 'demur' (to demur is to object to something).
'with' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mc2).
'demur'+'e'='DEMURE'
(Other definitions for demure that I've seen before include "Sedate" , "Pretending to be quiet and serious" , "Coy, shy" , "Affectedly coy and serious" , "Staid" .)