Proportion of allowance not finished (5)
I believe the answer is:
ratio
'proportion' is the definition.
(I know that proportion can be written as ratio)
'allowance not finished' is the wordplay.
'allowance' becomes 'ration' (I've seen this before).
'not finished' means to remove the last letter.
'ration' with its last letter removed is 'RATIO'.
'of' acts as a link.
(Other definitions for ratio that I've seen before include "Proportion between two numbers" , "Correlation" , "Proportional measure" , "Relative proportion" , "Proportional relation between two numbers" .)