Don't finish helping - it provides a sense of proportion (5)
I believe the answer is:
ratio
'proportion' is the definition.
(I know that proportion can be written as ratio)
'don't finish helping' is the wordplay.
'don't finish' means to remove the last letter.
'helping' becomes 'ration' (a ration is a serving or helping).
'ration' with its last letter removed is 'RATIO'.
'it provides a sense of' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ratio that I've seen before include "Relativity between two quantities" , "Mathematical relation of one thing to another" , "Proportional relationship" , "Relationship between two amounts" , "One to one - maybe" .)