Share out a tax rise (5)
I believe the answer is:
allot
'share out' is the definition.
(synonyms)
'a tax rise' is the wordplay.
'tax' becomes 'toll' (I've seen this in another clue).
'rise' is a reversal indicator.
'toll' reversed gives 'llot'.
'a'+'llot'='ALLOT'
(Other definitions for allot that I've seen before include "Distribute in portions" , "Dispense or distribute" , "Designate; sounds like much" , "Apportion (something to someone)" , "Dedicate" .)