Tax of the French emptied treasury (4)
I believe the answer is:
duty
'tax' is the definition.
(duty is a kind of tax)
'of the french emptied treasury' is the wordplay.
'of the french' becomes 'du' ('of the' in French).
'emptied' says to hollow out the word (remove centre letters).
'treasury' with its middle taken out is 'ty'.
'du'+'ty'='DUTY'
(Other definitions for duty that I've seen before include "Moral/legal obligation" , "Service - tax - office" , "Allegiance" , "charge" , "Not subject to customs tax" .)