Proportion of allowance docked (5)
I believe the answer is:
ratio
'proportion' is the definition.
(thesaurus)
'allowance docked' is the wordplay.
'allowance' becomes 'ration' (I've seen this before).
'docked' means to remove the last letter (to dock can mean to remove the tail of an animal).
'ration' with its last letter taken off is 'RATIO'.
'of' acts as a link.
(Other definitions for ratio that I've seen before include "figures in relationship" , "E.g. sin i" , "Expression of relationship" , "E.g. two to one" , "Proportional relation between two numbers" .)