Guarding of French barrier (7)
I believe the answer is:
defence
'guarding' is the definition.
(defence is a kind of guarding)
'of french barrier' is the wordplay.
'of french' becomes 'de'.
'barrier' becomes 'fence' (fence is a kind of barrier).
'de'+'fence'='DEFENCE'
(Other definitions for defence that I've seen before include "barricade" , "This department deals with national protection" , "Protection from danger" , "Case for the accused" , "See 12" .)