Completely self-obsessed king breaks fast? (7)
I believe the answer is:
allegro
'fast?' is the definition.
(similar in meaning)
'completely self-obsessed king breaks' is the wordplay.
'completely' becomes 'all' (synonyms).
'self-obsessed' becomes 'ego'.
'king' becomes 'R' (abbreviation of Latin rex).
'breaks' is an insertion indicator (inserted letters break into the word).
'ego' enclosing 'r' is 'egro'.
'all'+'egro'='ALLEGRO'
(Other definitions for allegro that I've seen before include "Austin" , "Musical movement using a brisk speed" , "Moving fast" , "In quick and lively style" , "In a brisk and lively manner" .)