Composer's grand fortune (5)
I believe the answer is:
gluck
'composer's' is the definition.
(I've seen this before)
'grand fortune' is the wordplay.
'grand' becomes 'g' (G is short for grand as in a thousand).
'fortune' becomes 'luck' (synonyms).
'g'+'luck'='GLUCK'
(Other definitions for gluck that I've seen before include "German composer of operas" , "Composer of Orfeo ed Eurydice" , "Eighteenth century German composer (Orpheus and Eurydice)" .)