Barrier keeps you in France: fact (5)
I believe the answer is:
datum
'fact' is the definition.
(singular form of 'data')
'barrier keeps you in france' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'keeps' indicates putting letters inside (inserted letters are kept inside).
'you in france' becomes 'tu' ('you' in French).
'dam' placed around 'tu' is 'DATUM'.
(Other definitions for datum that I've seen before include "Given item" , "bit of info" , "Single piece of information" , "Given fact" , "given to the Romans" .)