Ruin ground, parking inside (5)
I believe the answer is:
spoil
'ruin' is the definition.
(I've seen this before)
'ground parking inside' is the wordplay.
'ground' becomes 'soil' (synonyms).
'parking' becomes 'p' (abbreviation on road signs).
'inside' is an insertion indicator.
'soil' going around 'p' is 'SPOIL'.
(Other definitions for spoil that I've seen before include "Butcher" , "baby" , "Having dredged-up material" , "Mess up" , "decompose" .)