Large and small in a fantastic part of France (6)
I believe the answer is:
alsace
'part of france' is the definition.
('Alsace' is part of 'france')
'large and small in a fantastic' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'and' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'in' means one lot of letters goes inside another.
'fantastic' becomes 'ace' (ace can informally mean very good).
'l'+'s'='ls'
'a'+'ace'='aace'
'ls' inserted inside 'aace' is 'ALSACE'.
(Other definitions for alsace that I've seen before include "source of French wines" , "area of France" , "somewhere in France" , "Wine-producing area" , "French region bordering Germany and Switzerland" .)