Bell family, say, turned against Romeo (7)
I believe the answer is:
clanger
'bell' is the definition.
(I've seen this before)
'family say turned against romeo' is the wordplay.
'family' becomes 'clan' (I've seen this before).
'say' becomes 'eg' (both can mean 'for example').
'turned' shows that the letters should be reversed in order.
'against' is a charade indicator (letters next to each other).
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'eg' back-to-front is 'ge'.
'clan'+'ge'+'r'='CLANGER'
(Other definitions for clanger that I've seen before include "Drop this to make a blunder" , "Howler" , "noise that rings in the ear" , "Bad mistake - loud bell" , "Big blunder" .)