Completely self-obsessed Romeo breaks fast (7)
I believe the answer is:
allegro
'fast' is the definition.
(similar in meaning)
'completely self-obsessed romeo breaks' is the wordplay.
'completely' becomes 'all' (synonyms).
'self-obsessed' becomes 'ego'.
'romeo' becomes 'R' (phonetic alphabet: alpha, bravo, charlie etc.).
'breaks' indicates putting letters inside (inserted letters break into the word).
'ego' placed around 'r' is 'egro'.
'all'+'egro'='ALLEGRO'
(Other definitions for allegro that I've seen before include "'Brisk and lively, musically (7)'" , "Musically, in a lively fashion" , "In quick time" , "Brisk mus. speed" , "Musical movement using a brisk speed" .)