Energy after reduction is precious (4)
I believe the answer is:
cute
'precious' is the definition.
('cute' can be a synonym of 'precious')
'energy after reduction' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'after' means one lot of letters go next to another.
'reduction' becomes 'cut' (cut is a kind of reduction).
'e' put after 'cut' is 'CUTE'.
'is' acts as a link.
(Other definitions for cute that I've seen before include "Charming, quaint" , "Daintily pleasing" , "Sweet, attractive" , "Endearing" , "Precious" .)