Nothing but repetition? Rather regular repetition maybe! (12)
I believe the answer is:
alliteration
'rather regular repetition maybe' is the definition.
(alliterative phrase)
'nothing but repetition?' is the wordplay.
'nothing but' becomes 'all' (I've seen this in other clues).
'repetition?' becomes 'iteration' (iteration is a kind of repetition).
'all'+'iteration'='ALLITERATION'
(Other definitions for alliteration that I've seen before include "Poetic rhyming device using words with the same initial letter" , "Successive starts sounding similarly" , "Repeated first letters" , "Poetic ploy practised with panache!" , "verse technique" .)