That is the thing about some in France (2,3)
I believe the answer is:
id est
'that is' is the definition.
('id est' can be a synonym of 'that is')
'the thing about some in france' is the wordplay.
'the thing' becomes 'it'.
'about' means one lot of letters goes inside another.
'some in france' becomes 'des'.
'it' placed around 'des' is 'ID EST'.
(Other definitions for id est that I've seen before include "that is in Rome" , "ie (L)" , "In other words" , "Ie, in full" , "That is to say (Latin)" .)