Composer saw audience thrilled in Eroica initially (5)
I believe the answer is:
satie
'composer' is the definition.
(French composer Erik Satie)
'saw audience thrilled in eroica initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'saw audience thrilled in eroica' is 'SATIE'.
(Other definitions for satie that I've seen before include "Erik -, nineteenth to twentieth century French composer" , "Erik -----, French early minimalist composer" , "French composer of the Gymnopedies, d. 1925" , "scorer" , "French composer (Trois Gymnopedies)" .)