Parking site - terrible slope (5)
I believe the answer is:
piste
'slope' is the definition.
(I have seen 'slippery slope' mean 'piste' so perhaps 'slope' could also mean 'piste')
'parking site terrible' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'terrible' indicates anagramming the letters.
'site' is an anagram of 'iste'.
'p'+'iste'='PISTE'
(Other definitions for piste that I've seen before include "Skiing course" , "Ski slope" , "Winter sports course" , "Ski trail" , "Skiing trail" .)