It's attractive to reduce energy (4)
I believe the answer is:
cute
'it's attractive' is the definition.
(similar in meaning)
'reduce energy' is the wordplay.
'reduce' becomes 'cut' (synonyms).
'energy' becomes 'e' (physics symbol as in E=mc2).
'cut'+'e'='CUTE'
'to' acts as a link.
(Other definitions for cute that I've seen before include "Small, delightful" , "Precious" , "Quaintly pleasing" , "Attractive and quaint (4" , "Clever - attractive" .)