Allowance cut in proportion (5)
I believe the answer is:
ratio
'in proportion' is the definition.
(I've seen this before)
'allowance cut' is the wordplay.
'allowance' becomes 'ration' (I've seen this before).
'cut' means to remove the last letter.
'ration' with its final letter removed is 'RATIO'.
(Other definitions for ratio that I've seen before include "Measure expressed as a proportion" , "Relation between two things, numbers say" , "Comparative size" , "Quotient of two numbers" , "Quotient, proportion" .)