Earmark a tax to be raised (5)
I believe the answer is:
allot
'earmark' is the definition.
(thesaurus)
'a tax to be raised' is the wordplay.
'tax' becomes 'toll' (I've seen this in other clues**).
'to be raised' says the letters should be written in reverse.
'toll' in reverse letter order is 'llot'.
'a'+'llot'='ALLOT'
(Other definitions for allot that I've seen before include "split up" , "Give as a share, or a task to be done" , "Earmark" , "appropriate" , "Ration" .)