Adult prepared tortellini -- a popular pasta, perhaps? (12)
I believe the answer is:
alliteration
'popular pasta perhaps?' is the definition.
('popular pasta' is an example of alliteration)
'adult prepared tortellini a' is the wordplay.
'adult' becomes 'a' (abbreviation).
'prepared' indicates an anagram.
'tortellini'+'a'='tortellinia'
'tortellinia' anagrammed gives 'lliteration'.
'a'+'lliteration'='ALLITERATION'
(Other definitions for alliteration that I've seen before include "verse technique" , "Peter practised poetic ploy perfectly" , "Poetic ploy practised with panache!" , "Repeated first letters" , "Successive starts sounding similarly" .)