Proportion of allowance unfinished (5)
I believe the answer is:
ratio
'proportion' is the definition.
(thesaurus)
'allowance unfinished' is the wordplay.
'allowance' becomes 'ration' (I've seen this before).
'unfinished' means to remove the last letter.
'ration' with its last letter taken off is 'RATIO'.
'of' acts as a link.
(Other definitions for ratio that I've seen before include "Relativity between two quantities" , "Relative proportion" , "nine-to-five, say" , "eg 1:3" , "Quotient, proportion" .)