Distribute key tax returns (5)
I believe the answer is:
allot
'distribute' is the definition.
(allotting is a kind of distributing)
'key tax returns' is the wordplay.
'key' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'tax' becomes 'toll' (I've seen this in other clues).
'returns' shows that the letters should be reversed in order.
'toll' backwards is 'llot'.
'a'+'llot'='ALLOT'
(Other definitions for allot that I've seen before include "budget" , "Assign a share" , "Hand out" , "Assign or distribute" , "Dish out" .)