Tender on open river (8)
I believe the answer is:
overture
'tender' is the definition.
(thesaurus)
'on open river' is the wordplay.
'on' says to put letters next to each other.
'open' becomes 'overt' ('overt' is associated in meaning with 'open'**).
'river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'overt'+'ure' is 'OVERTURE'.
(Other definitions for overture that I've seen before include "10 {COMPOSITION]" , "Pre-curtain-up piece" , "Opera opening" , "used in attempt to seduce?" , "Orchestral opener" .)