Trio not right for you once (4)
I believe the answer is:
thee
'you once' is the definition.
(historical second person pronoun)
'trio not right' is the wordplay.
'trio' becomes 'three' ('three' can be a synonym of 'trio').
'not' suggests deleting specific letters.
'right' becomes 'r' (common abbreviation).
'three' with 'r' taken away is 'THEE'.
'for' is the link.
(Other definitions for thee that I've seen before include "You in the olden days" , "'U'" , "Quaker 'you'" , "Poetically you" , "Archaic form of 'you'" .)