Clue: Repetition is, after all, Peter Piper's speciality (12)
Answer: ALLITERATION
Explanation: Repetition is, after all, Peter Piper's speciality (12)
There is alliteration in "after all" and "Peter Piper". 
The clue also indicates that there may be something else going on – ITERATION after ALL, but I don't see the "iteration" = "Peter Piper" bit?