A trio composed in proportion (5)
I believe the answer is:
'in proportion' is the definition.
(I've seen this before)
'a trio composed' is the wordplay.
'composed' indicates an anagram.
'atrio' is an anagram of 'RATIO'.
(Other definitions for ratio that I've seen before include "Proportion, relationship" , "Eg, three to one" , "Expression of relationship" , "Relationship of numbers" , "Mathematical relation of one thing to another" .)