Quibble about parking behind the saloon? (4)
I believe the answer is:
carp
'quibble' is the definition.
(I've seen this in another clue)
'parking behind the saloon?' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'behind' is a charade indicator (letters next to each other).
'the saloon?' becomes 'car' (type of car).
'p' after 'car' is 'CARP'.
'about' acts as a link.
(Other definitions for carp that I've seen before include "Fish often kept in ponds" , "praise? The opposite" , "Pond fish" , "Fish; nag pettily" , "ish" .)