Talk of Romeo appearing in a sort of toga! (5)
I believe the answer is:
argot
'talk' is the definition.
(I have seen 'Talk of thieves' mean 'argot' so perhaps 'talk' could also mean 'argot')
'romeo appearing in a sort of toga' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'appearing in' is an insertion indicator.
'a sort of' indicates anagramming the letters.
'toga' anagrammed gives 'agot'.
'r' going within 'agot' is 'ARGOT'.
'of' acts as a link.
(Other definitions for argot that I've seen before include "Slang for a groat (5)" , "Cant" , "Ancient Greek coin" , "Special jargon of a group" , "Jargon or slang (of a group)" .)