Genius, master, ruined by love (7)
I believe the answer is:
maestro
'genius' is the definition.
(I've seen this before)
'master ruined by love' is the wordplay.
'ruined' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'love' becomes 'o' (love means zero in tennis).
'master' anagrammed gives 'maestr'.
'maestr'+'o'='MAESTRO'
(Other definitions for maestro that I've seen before include "Some art (anag.)" , "Artist of consummate skill: conductor" , "Are most in favour of a foreign master?" , "Gifted performer" , "hotshot" .)