Good French wine inside a stronghold (7)
I believe the answer is:
bastion
'a stronghold' is the definition.
(bastion is a kind of stronghold)
'good french wine inside' is the wordplay.
'good french' becomes 'bon' ('good' in French).
'wine' becomes 'asti' (Italian wine).
'inside' means one lot of letters goes inside another.
'bon' enclosing 'asti' is 'BASTION'.
(Other definitions for bastion that I've seen before include "part of stronghold" , "Defence - in boats (anag)" , "Obtains in fortified place or stronghold" , "A projecting fortification to increase angle of fire" , "staunch supporter" .)