Numerical relationship; proportion (5)
I believe the answer is:
ratio
This clue is a double definition.
'numerical relationship' is the first definition.
(I've seen this in another clue)
'proportion' is the second definition.
(I know that proportion can be written as ratio)
(Other definitions for ratio that I've seen before include "A trio (anag)" , "One-to-one, maybe" , "Relative magnitude" , "Eg, three to one" , "Proportional relationship" .)