Clapton provided voices for Lumpy Gravy, though, contrary to what's been said in the liner notes, he isn't actually present on the recording. In a backstage interview in Copenhagen, March 3, 1976, Zappa said that the mention of Clapton on that album is a mistake. 
During The Frank Zappa Interview Picture Disk, pt.2 interview Zappa said the following about Clapton: "I know Eric, I haven't seen him in years and years. There's another guy who shouldn't use drugs."
In an interview with Rolling Stone from October 17, 2015 Ahmet Zappa talked about Alex Winter's documentary Who the F*@% is Frank Zappa? and revealed he acquired film footage of Frank jamming with Eric Clapton in 1967 and was blown away by the quality of the footage.