Proportion of allowance cut (5)
I believe the answer is:
ratio
'proportion' is the definition.
(thesaurus)
'allowance cut' is the wordplay.
'allowance' becomes 'ration' (I've seen this before).
'cut' means to remove the last letter.
'ration' with its last letter taken away is 'RATIO'.
'of' is the link.
(Other definitions for ratio that I've seen before include "A trio (anag)" , "Relationship of numbers" , "Proportion, relation" , "Proportional measure" , "Measure expressed as a proportion" .)