Noble gas returns are largely nonexempt (5)
I believe the answer is:
xenon
'noble gas' is the definition.
(I know that xenon is a type of noble gas)
'returns are largely nonexempt' is the wordplay.
'XENON' is hidden back-to-front within the letters.
But, I'm not sure how this is indicated.
Can you help me to learn more?
(Other definitions for xenon that I've seen before include "Heavy, colourless and odourless gas" , "Gaseous element, Xe" , "Inert gas, Xe" , "One of the noble set" , "Chemical element" .)