It's really annoying me that I can't remember it! It'll be incredibly vague though, sorry... I've seen a clip from a musical on YouTube, and would like to watch it again. The song might be a standard as well, but I'm not sure!
It's two people singing on a bridge, at night. I think it could be an MGM film, just because I know it's a soundstage (the stars twinkle in a pattern!). The film is in colour. It's shot from on the bridge with them.
It's a love song, but a gentle rather than triumphant one, if that makes sense. I'm almost certain it isn't Rodgers and Hammerstein.
Unhelpfully I can't properly 'see' the actor or the actress!
Much 
and 
if anybody can get it! I'll keep trying to think as well.