Parking behind river is extreme (4)
I believe the answer is:
deep
'extreme' is the definition.
(synonyms)
'parking behind river' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'behind' is a charade indicator (letters next to each other).
'river' becomes 'dee' (River Dee).
'p' put after 'dee' is 'DEEP'.
'is' is the link.
(Other definitions for deep that I've seen before include "Profound - cunning - sea - bass" , "hard to understand" , "Intense; low (in pitch)" , "Profound - engrossed" , "(Of sound) low in pitch" .)