Charges at retreating rabble — half vanished (6)
I believe the answer is:
tariff
'charges' is the definition.
(a tariff is a system of charges)
'at retreating rabble half vanished' is the wordplay.
'retreating' shows that the letters should be reversed in order.
'rabble' becomes 'riffraff' (both can mean ordinary people).
'half vanished' means to take half the letters.
'at' reversed gives 'ta'.
'riffraff' with half the letters taken is 'riff'.
'ta'+'riff'='TARIFF'
(Other definitions for tariff that I've seen before include "Rate" , "Customs duty" , "Duty; table of charges" , "Fees schedule" , "List of prices or charges" .)