Complainer Romeo in bad cough (6)
I believe the answer is:
grouch
'complainer' is the definition.
(thesaurus)
'romeo in bad cough' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'bad' indicates anagramming the letters.
'cough' with letters rearranged gives 'gouch'.
'r' placed inside 'gouch' is 'GROUCH'.
(Other definitions for grouch that I've seen before include "carp" , "Bellyache" , "Grumble - complain" , "Complain(ing person)" , "one known for grumbling" .)