Crossword Genius

A trio composed in proportion (5)

Ross

I believe the answer is:

ratio

'in proportion' is the definition.
(I've seen this before)

'a trio composed' is the wordplay.
'composed' indicates an anagram.
'a'+'trio'='atrio'
'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" .)

I've seen this clue in the Evening Standard.
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