Musical composer left note inside organ (5)
I believe the answer is:
elgar
'musical composer' is the definition.
(Edward Elgar)
'left note inside organ' is the wordplay.
'left' becomes 'l' (common abbreviation).
'note' becomes 'g' (musical note**).
'inside' means one lot of letters goes inside another.
'organ' becomes 'ear' (ear is a kind of organ).
'l'+'g'='lg'
'lg' inserted within 'ear' is 'ELGAR'.
(Other definitions for elgar that I've seen before include "Score-person" , "Scorer" , "Glare critically at the British composer" , "Glare at great English composer" , "man of note" .)