Fixed allowance almost in proportion (5)
I believe the answer is:
ratio
'in proportion' is the definition.
(I've seen this before)
'fixed allowance almost' is the wordplay.
'fixed allowance' becomes 'ration' (a fixed quota given out to someone).
'almost' means to remove the last letter (most of the word but not all of it).
'ration' with its final letter taken off is 'RATIO'.
(Other definitions for ratio that I've seen before include "3 relation" , "E.g. sin i" , "Relationship between two numerical quantities" , "Comparative size" , "Quotient of two numbers" .)