Short of energy, resistance overwhelmed by United (5)
I believe the answer is:
tired
'short of energy' is the definition.
(tired can mean lacking in energy)
'resistance overwhelmed by united' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'overwhelmed by' indicates putting letters inside (inserted letters get absorbed or overwhelmed).
'united' becomes 'tied' (similar in meaning).
'r' put within 'tied' is 'TIRED'.
(Other definitions for tired that I've seen before include "Beat" , "Depleted of energy" , "Physically or mentally weary" , "Are American cars so" , "Worn-out" .)