Apportions when claiming tax back (6)
I believe the answer is:
allots
'apportions' is the definition.
(I've seen this before)
'when claiming tax back' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'claiming' means one lot of letters goes inside another.
'tax' becomes 'toll' (I've seen this in another clue).
'back' shows that the letters should be reversed in order.
'toll' backwards is 'llot'.
'as' going around 'llot' is 'ALLOTS'.
(Other definitions for allots that I've seen before include "Apportions or assigns" , "Dispenses or distributes" , "Allows to have" , "Distributes, gives as share" , "Gives, apportions" .)