The most common and most likely answer to this crossword clue is the 12 letter word ALLITERATION
I've seen this crossword clue in The Guardian
A
L
L
I
T
E
R
A
T
I
O
N
More clues leading to the result ALLITERATION
- Poetic technique nothing but repeats
- Nothing but repetition? Rather regular repetition, perhaps
- A feature of double dosing doing everything with repetition
- Literary effect left one boring revision
- Initial repetition is only repetition after all
- Term to typify “Troubled toenail trial”?
- Repetition is, after all, Peter Piper’s speciality
- Finishes off feta and fennel, fat-free food?
- One line rejected in adaptation of Nicholas Nickleby, say
- Initial letters of Shakespeare sonnet, say, supply solution?
Buddy explains!

The straight part
We know that "Initial repetition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case initial repetition = alliteration.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ALLITERATION mean?
ALLITERATION noun- The repetition of consonant sounds at the beginning of two or more words immediately succeeding each other, or at short intervals.
- The recurrence of the same letter in accented parts of words, as in Anglo-Saxon alliterative meter.