Allowance is curtailed in proportion (5)
I believe the answer is:
ratio
'in proportion' is the definition.
(I've seen this before)
'allowance is curtailed' is the wordplay.
'allowance' becomes 'ration' (I've seen this before).
'is curtailed' means to remove the last letter.
'ration' with its last letter taken off is 'RATIO'.
(Other definitions for ratio that I've seen before include "3 relation" , "nine-to-five, say" , "Relative magnitude" , "One to one - maybe" , "Eg, three to one" .)