Support for parking (4)
I believe the answer is:
prop
'support' is the definition.
(prop up can mean to support)
'for parking' is the wordplay.
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'parking' becomes 'p' (abbreviation on road signs).
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "Stay" , "A rigid supprot or piece of satge furniture (4)" , "Support to keep from falling" , "Back up" , "Support - item on stage" .)