Envoy using the French opening (6)
I believe the answer is:
legate
'envoy' is the definition.
(I know that legate is a type of envoy)
'the french opening' is the wordplay.
'the french' becomes 'le' ('the' in French).
'opening' becomes 'gate' (a gate is an opening or entrance).
'le'+'gate'='LEGATE'
'using' is the link.
(Other definitions for legate that I've seen before include "Envoy representing the Pope" , "Envoy, maybe papal" , "Papal ambassador" , "Late, eg. for the papal representative" , "Emissary representing the Pope" .)