Barrier restricts you in France - fact (5)
I believe the answer is:
datum
'fact' is the definition.
(I've seen this before)
'barrier restricts you in france' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'restricts' is an insertion indicator.
'you in france' becomes 'tu' (I've seen this in other clues).
'dam' placed around 'tu' is 'DATUM'.
(Other definitions for datum that I've seen before include "Something given" , "Premiss, thing taken for granted" , "Given fact" , "Basic fact" , "Given point or position" .)