Good — find out Romeo is the one complaining (8)
I believe the answer is:
grumbler
'the one complaining' is the definition.
'good find out romeo' is the wordplay.
'good' becomes 'g' (abbreviation).
'find out' becomes 'rumble' ('rumble' can mean to discover a scheme).
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'g'+'rumble'+'r'='GRUMBLER'
'is' is the link.
(Other definitions for grumbler that I've seen before include "One who complains" , "Persistent complainer" .)