Six, initially, on the French instrument (5)
I believe the answer is:
viola
'instrument' is the definition.
(viola is a kind of instrument)
'six initially on the french' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'initially' suggests taking the first letters.
'the french' becomes 'la' (one of the French words for 'the').
The first letter of 'on' is 'o'.
'vi'+'o'+'la'='VIOLA'
(Other definitions for viola that I've seen before include "Pansy-like plant" , "Bowed instrument slightly larger than a violin" , "Bloomer" , "Twelfth Night heroine" , "Plant; instrument" .)