Feature of Love's Labour's Lost nothing but repetition (12)
I believe the answer is:
alliteration
'feature of love's labour's lost' is the definition.
('Love's Labour's Lost' is alliterative)
'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 "Successive starts sounding similarly" , "Poetic rhyming device using words with the same initial letter" , "verse technique" , "Poetic ploy practised with panache!" , "Repeated first letters" .)