Short rest improved energy (5)
I believe the answer is:
terse
'short' is the definition.
('terse' can be a synonym of 'short')
'rest improved energy' is the wordplay.
'improved' indicates anagramming the letters.
'energy' becomes 'e' (physics symbol as in E=mc2**).
'rest' anagrammed gives 'ters'.
'ters'+'e'='TERSE'
(Other definitions for terse that I've seen before include "Short and to the point" , "Short and sharp" , "Short and succinct" , "Curtly brief" , "precise" .)