Coastal city below Wash (7)
I believe the answer is:
launder
'wash' is the definition.
(I know that wash can be written as launder)
'coastal city below' is the wordplay.
'coastal city' becomes 'LA' (Los Angeles).
'below' becomes 'under' (synonyms).
'la'+'under'='LAUNDER'
(Other definitions for launder that I've seen before include "Perhaps decriminalise" , "Make illegal (money) legitimate" , "Act illegally" , "Trough" , "clean habits" .)