Unit working with energy (3)
I believe the answer is:
one
'unit' is the definition.
('one' can be a synonym of 'unit')
'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 "Single person or thing" , "Single and undivided" , "United" , "Leading cardinal" , "Sole; number" .)