A trio's broken relationship (5)
I believe the answer is:
ratio
'relationship' is the definition.
(relationship between two numbers)
'a trio's broken' is the wordplay.
'broken' indicates anagramming the letters.
'a'+'trio'='atrio'
'atrio' anagrammed gives 'RATIO'.
(Other definitions for ratio that I've seen before include "3 relation" , "allowance" , "Correlation" , "Comparative measure" , "Relation between two things, numbers say" .)