'Mean as custard' for 'keen as mustard', say (10)
I believe the answer is:
spoonerism
(Other definitions for spoonerism that I've seen before include ""A lopping of sweaters", for example" , "Burly chassis for Shirley Bassey, say?" , "Verbal exchange of leading characters" , "Regular announcement by old clergyman" , "A slip of the tongue? -- no promises (anag)" .)