Nicer trio composed as a yardstick (9)
I believe the answer is:
criterion
'a yardstick' is the definition.
(thesaurus)
'nicer trio composed' is the wordplay.
'composed' indicates anagramming the letters.
'nicer'+'trio'='nicertrio'
'nicertrio' is an anagram of 'CRITERION'.
'as' acts as a link.
(Other definitions for criterion that I've seen before include "Judging standard" , "Standard; model" , "Standard by which something can be judged" , "Standard, reference point" , "Touchstone" .)