Reduced energy? Charming! (4)
I believe the answer is:
cute
'charming' is the definition.
(I've seen this in other clues)
'reduced energy?' is the wordplay.
'reduced' becomes 'cut' (similar in meaning).
'energy?' becomes 'e' (physics symbol as in E=mcu).
'cut'+'e'='CUTE'
(Other definitions for cute that I've seen before include "Charming, quaint" , "Attractive and quaint (4" , "Quaintly pleasing" , "Taking" , "Lovable" .)