A trio involved in correspondence (5)
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" .)