A trio composed in proportion (5)
I believe the answer is:
ratio
'in proportion' is the definition.
(I've seen this before)
'a trio composed' is the wordplay.
'composed' is an anagram indicator.
'a'+'trio'='atrio'
'atrio' anagrammed gives '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" .)