Romeo abandoning trio's second person once (4)
I believe the answer is:
thee
'second person once' is the definition.
(historical second-person pronoun)
'romeo abandoning trio's' is the wordplay.
'romeo' becomes 'R' (phonetic alphabet: alpha, bravo, charlie etc.).
'abandoning' indicates named letters should be taken away.
'trio' becomes 'three' (synonyms).
'three' with 'r' taken away is 'THEE'.
(Other definitions for thee that I've seen before include "Archaic form of 'you'" , "Quaker 'you'" , "Poetically you" , "Old objective second person singular" , "You (arch.)" .)