A trio involved with proportional representation (5)
I believe the answer is:
ratio
'proportional representation' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'a trio involved' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'a'+'trio'='atrio'
'atrio' is an anagram of 'RATIO'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for ratio that I've seen before include "A trio (anag)" , "nine-to-five, say" , "E.g. two to one" , "Proportion, relation" , "Percentage" .)