Fatal notes upset lady (6)
I believe the answer is:
deadly
'fatal' is the definition.
(similar in meaning)
'notes upset lady' is the wordplay.
'notes' becomes 'de' (two musical notes).
'upset' is an anagram indicator.
'lady' is an anagram of 'adly'.
'de'+'adly'='DEADLY'
(Other definitions for deadly that I've seen before include "No small bore" , "sniper aspires to be" , "Lethal, capable of killing" , "Lethal, capable of causing fatality" , "There are seven such sins" .)