Possible Clues:

• Always left in the red on board
• Red, but left for a not heavy drinker
• Not a heavy red wine that's left
• Sounds as if one may take it as read that's it's left on board
• The wine that's not been fortified gets left on board
• Having so little body, the wine gets left on board
• Not perhaps for the heavy drinker that gets left on board
• Left on board with the red wine that's not heavy
• Not a heavy red wine is all that's left at sea
• It's not for a heavy drinker, and not on 15 down