Playing a trio in proportion (5)
I believe the answer is:
ratio
'in proportion' is the definition.
(I've seen this before)
'playing a trio' is the wordplay.
'playing' indicates an anagram.
'a'+'trio'='atrio'
'atrio' with letters rearranged gives 'RATIO'.
(Other definitions for ratio that I've seen before include "One to one - maybe" , "Measure of two relative sizes" , "Mathematical relationship" , "Relativity between two quantities" , "five to four, perhaps" .)