Possible Clues:

• Singing nymph
• Just a myth to have one round your neck and on the rocks (7)
• Siren with secret knowledge gets floral tribute
• Learning flower garland is for the enchantress
• Great Rock on the Rhine where 37 across lived
• Would the girl be untruthful, by the sound of it, on the Rhine?
• Rhine rock with legendary siren
• Attractive woman flowers after learning
• Legendary siren luring Rhine boatmen to destruction
• Siren in mythology with garland on