Divide a tax on motorists up (5)
I believe the answer is:
allot
'divide' is the definition.
(thesaurus)
'a tax on motorists up' is the wordplay.
'tax on motorists' becomes 'toll' (I can't explain this - if you can you should believe this answer much more).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'toll' in reverse letter order is 'llot'.
'a'+'llot'='ALLOT'
Can you help me to learn more?
(Other definitions for allot that I've seen before include "Parcel out, assign" , "Distribute, give as share" , "Designate; sounds like much" , "Distribute in portions" , "budget" .)