When Velma said clue, she didn't mean you, Scooby.
4 seconds sound clip from Scooby-Doo, Where Are You! - What a Night for a Knight (S1E1).
You can hear this line at 00:14:14.641 in the Blu-ray version of the episode.
Quote context
[...]
- Scooby-Doo! Scooby-Dooby-Doo!
- I tell you, the Black Knight is alive and roaming through the hallways.
- If we could only find a clue.
- When Velma said clue, she didn't mean you, Scooby.
- Right! It's right down here.
- There's no picture missing, Shaggy.
- Well, if that's blood... That's a clue.
- It's paint. And look... A trail of it leads down the hallway. Come on, follow that paint.
[...]