Correlation or arrangement of a trio (5)
I believe the answer is:
ratio
'correlation' is the definition.
(I've seen this before)
'arrangement of a trio' is the wordplay.
'arrangement of' indicates anagramming the letters.
'a'+'trio'='atrio'
'atrio' is an anagram of 'RATIO'.
'or' acts as a link.
(Other definitions for ratio that I've seen before include "Proportion between two numbers" , "Mathematical relation of one thing to another" , "Relationship found in a riot" , "Proportional relationship" , "Mathematical relationship" .)