Perturbed re snow, go downhill (6)
I believe the answer is:
worsen
'go downhill' is the definition.
(I've seen this before)
'perturbed re snow' is the wordplay.
'perturbed' indicates anagramming the letters.
're'+'snow'='resnow'
'resnow' anagrammed gives 'WORSEN'.
(Other definitions for worsen that I've seen before include "Become less well" , "go downhill" , "Become more inferior" , "Deteriorate for owners" , "Aggravate" .)