Natural energy wasted, say (4)
I believe the answer is:
easy
'natural' is the definition.
(similar in meaning)
'energy wasted say' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'wasted' indicates an anagram.
'say' is an anagram of 'asy'.
'e'+'asy'='EASY'
(Other definitions for easy that I've seen before include "Requiring little effort" , "Frederick Marryat's midshipman" , "Tranquil" , "Achieved without effort" , "careful!" .)