Maestro left in fury when shown up (5)
I believe the answer is:
elgar
'maestro' is the definition.
(composer Edward Elgar)
'left in fury when shown up' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'fury' becomes 'rage' (synonyms).
'when shown up' says the letters should be written in reverse.
'l' put inside 'rage' is 'ragle'.
'ragle' in reverse letter order is 'ELGAR'.
(Other definitions for elgar that I've seen before include "The 18 [APOSTLES] creator" , "Sir Edward -, English composer" , "[NIMROD] 's creator" , "Glare critically at the British composer" , "Scorer" .)