Open a French window, at the start, before musical performance (6)
I believe the answer is:
unwrap
'open' is the definition.
(I've seen this before)
'a french window at the start before musical performance' is the wordplay.
'a french' becomes 'un' ('a' in French).
'at the start' says to take the initial letters.
'before' says to put letters next to each other.
'musical performance' becomes 'rap' (rapping is a type of musical performance).
The first letter of 'window' is 'w'.
'un'+'w'+'rap'='UNWRAP'
(Other definitions for unwrap that I've seen before include "Remove coverings from (something)" , "Undo" , "Take the covering off, a parcel say" , "Open (a present)" , "Remove packaging from" .)