Discourage crazy reverse parking (4)
I believe the answer is:
damp
'discourage' is the definition.
(thesaurus)
'crazy reverse parking' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'reverse' says the letters should be written in reverse.
'parking' becomes 'p' (abbreviation on road signs).
'mad' backwards is 'dam'.
'dam'+'p'='DAMP'
(Other definitions for damp that I've seen before include "Slightly moist" , "Moist air" , "Clammy" , "Somewhat moist" , "dripping" .)