Numerical relationship (5)
I believe the answer is:
ratio
'numerical relationship' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ratio that I've seen before include "Relationship found in a riot" , "3 relation" , "Eg, 4 to 3" , "A trio (anag)" , "Relationship between two numerical quantities" .)