Impairs the perfection of the planet (4)
I believe the answer is:
mars
This might be a double definition.
'impairs' is the first definition.
('mar' can be a synonym of 'impair')
'perfection of the planet' is the second definition.
'Mars' can be an answer for 'planet' (Mars is a planet). I am not sure about the 'perfection of the' bit.
'the' is the link.
Can you help me to learn more?
(Other definitions for mars that I've seen before include "bar....that's sweet!" , "classical god" , "Famous Texas mission" , "Possibly pollutes" , "Next planet out" .)