A trio's awful relationship (5)
I believe the answer is:
ratio
'relationship' is the definition.
(I've seen this before)
'a trio's awful' is the wordplay.
'awful' indicates an anagram.
'a'+'trio'='atrio'
'atrio' anagrammed gives 'RATIO'.
(Other definitions for ratio that I've seen before include "Proportion, relation" , "Relation between two amounts" , "9 to 5, say?" , "E.g. two to one" , "Relationship of numbers" .)