Individual working with energy (3)
I believe the answer is:
one
'individual' is the definition.
'working with energy' is the wordplay.
'working' becomes 'on'.
'with' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mc2).
'on'+'e'='ONE'
(Other definitions for one that I've seen before include "I can mean this" , "1" , "Unit" , "This is singular" , "Lowest cardinal number" .)