'Rubber baby buggy bumper', for example (6-7)
I believe the answer is:
tongue-twister
(Other definitions for tongue-twister that I've seen before include "It's hard to say!" , "Sequence of words difficult to say" , "Peter Piper picked a peck of pickled pepper, for instance" , "Phrase that is difficult to say properly" , ""Red lorry, yellow lorry", for example" .)