Miley Cyrus — New Clue In Weed Mystery … Courtesy of Adam Lambert

Only Adam Lambert really knows for sure if that lit cigar he passed Miley Cyrus at his Halloween party contained the devil”s weed … and over the weekend, he dropped a pretty big clue.

Lambert was at LAX when a photog asked point-blank about the photo our sources posted, showing the former “Idol” superstar handing Miley something that looks (and probably was) a big, fat blunt.

You gotta see Lambert masterfully bob, weave and deflect when asked about it — but then came the big moment … when the photog said, “If it was a joint, say nothing.”

The reaction pretty much says it all.  

Now excuse us … we have some Funyuns to eat.