Superfluous quantity to be found in revised rate (5)
I believe the answer is:
extra
'superfluous' is the definition.
(I know that superfluous can be written as extra)
'quantity to be found in revised rate' is the wordplay.
'quantity' becomes 'x' (X is an example).
'to be found in' means one lot of letters goes inside another.
'revised' is an anagram indicator.
'rate' anagrammed gives 'etra'.
'x' going inside 'etra' is 'EXTRA'.
(Other definitions for extra that I've seen before include "someone acting in a crowd" , "Bye or wide, for example" , "Surplus to requirements" , "No-ball or wide, perhaps?" , "Surcharge" .)