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