Jetty parking that is next to river (4)
I believe the answer is:
pier
'jetty' is the definition.
(I've seen this before)
'parking that is next to river' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'that is' becomes 'ie' (synonyms).
'next to' means one lot of letters go next to another.
'river' becomes 'r'.
'p'+'ie'+'r'='PIER'
(Other definitions for pier that I've seen before include "Structure extending seawards" , "Buttress; landing place" , "Coastal platform" , "Mole, wall between windows" , "seaside location" .)