Approach public on river (8)
I believe the answer is:
overture
'approach' is the definition.
(synonyms)
'public on river' is the wordplay.
'public' becomes 'overt' (associated in meaning).
'on' means one lot of letters go next to another.
'river' becomes 'Ure' (river in Yorkshire).
'overt'+'ure'='OVERTURE'
(Other definitions for overture that I've seen before include "Opera opening" , "Item of music" , "Opening music" , "Friendly approach or orchestral prelude" , "offer" .)