Crossword Genius

A trio involved in correspondence (5)

Ross

I believe the answer is:

ratio

'correspondence' is the definition.
(thesaurus)

'a trio involved' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'a'+'trio'='atrio'
'atrio' anagrammed gives 'RATIO'.

'in' is the link.

(Other definitions for ratio that I've seen before include "Comparative figure" , "Mathematical relation of one thing to another" , "Relative proportion" , "Relative magnitudes of two quantities" , "Relationship between two numerical quantities" .)

I've seen this clue in the Evening Standard.
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