What Peter Piper did, for example, is hard to say (6-7)
I believe the answer is:
tongue-twister
I believe this clue is a double definition.
'what peter piper did for example' is the first definition.
('Peter Piper picked a peck of pickled peppers')
'is hard to say' is the second definition.
(a tongue-twister is hard to say)
(Other definitions for tongue-twister that I've seen before include "Peter Piper picked a peck of pickled pepper, for instance" , ""Red lorry, yellow lorry", for example" , ""I wish to wash my Irish wristwatch", perhaps" , "It's hard to say!" , "Sequence of words difficult to say" .)