Duty of the French army finally protecting Vatican City (4)
I believe the answer is:
levy
'duty' is the definition.
(I've seen this before)
'the french army finally protecting vatican city' is the wordplay.
'the french' becomes 'le' ('the' in French).
'finally' suggests the final letters.
'protecting' indicates putting letters inside (inserted letters are held or protected).
'vatican city' becomes 'v'.
The last letter of 'army' is 'y'.
'le'+'y'='ley'
'ley' enclosing 'v' is 'LEVY'.
'of' acts as a link.
(Other definitions for levy that I've seen before include "Charge imposed and collected" , "Raise (money, troops)" , "Impose tax/fee/fine" , "An imposed tax or duty" , "Imposition of charge" .)