More natural noise made by a cat full of energy (5)
I believe the answer is:
purer
'more natural' is the definition.
(pure can mean natural or unadulterated)
'noise made by a cat full of energy' is the wordplay.
'noise made by a cat' becomes 'purr' (cats purr).
'full of' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'purr' going around 'e' is 'PURER'.
(Other definitions for purer that I've seen before include "cleaner" , "More chaste" , "Less adulterated" , "more clean" , "Less immoral" .)