Parade ground cleared of harmful radon; an improvement (7)
I believe the answer is:
upgrade
'improvement' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for upgrade that I've seen before include "Improve equipment or promote" , "Raise to higher standard" , "Raise to higher class (air travel)" , "Bonus for plane passenger" , "Get better software" .)