Mourn Romeo with bird (6)
I believe the answer is:
regret
'mourn' is the definition.
(I've seen this before)
'romeo with bird' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'with' says to put letters next to each other.
'bird' becomes 'egret' (egret is a kind of bird**).
'r'+'egret'='REGRET'
(Other definitions for regret that I've seen before include "Subsidiary theorem" , "Lament" , "Wish undone" , "Be sorry for, wish it hadn't happened" , "Feel repentant" .)