A trio breaks up a relationship (5)
I believe the answer is:
ratio
'a relationship' is the definition.
(relationship between two numbers)
'a trio breaks up' is the wordplay.
'breaks up' indicates an anagram.
'a'+'trio'='atrio'
'atrio' with letters rearranged gives 'RATIO'.
(Other definitions for ratio that I've seen before include "Relationship between two amounts" , "Proporation, quantitative relation" , "Correlation" , "Comparative size" , "3 relation" .)