Pet is rolling on a 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')
'pet is rolling' is the wordplay.
'rolling' indicates anagramming the letters.
'pet'+'is'='petis'
'petis' with letters rearranged gives 'PISTE'.
'on a' acts as a link.
(Other definitions for piste that I've seen before include "A ski run of compacted snow" , "Ski trail" , "Ski track" , "Skiing trail" , "Ski slope" .)