Probability expression (5)
I believe the answer is:
ratio
'probability expression' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ratio that I've seen before include "Eg, three to one" , "Comparative size" , "A trio (anag)" , "Proportion of one thing to another" , "9 to 5, say?" .)