Open a French type of sandwich with nothing in it (6)
I believe the answer is:
unbolt
'open' is the definition.
(unbolting is a kind of opening)
'a french type of sandwich with nothing in it' is the wordplay.
'a french' becomes 'un' ('a' in French).
'type of sandwich' becomes 'blt'.
'with nothing' becomes 'o' (I've seen this in other clues).
'in it' means one lot of letters goes inside another.
'blt' enclosing 'o' is 'bolt'.
'un'+'bolt'='UNBOLT'
(Other definitions for unbolt that I've seen before include "Open" , "Unlock" .)