Old coin Romeo found in butter (5)
I believe the answer is:
groat
'old coin' is the definition.
(I've seen this before)
'romeo found in butter' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'found in' means one lot of letters goes inside another.
'butter' becomes 'goat' (a goat is a butt-er).
'r' put within 'goat' is 'GROAT'.
(Other definitions for groat that I've seen before include "Medieval coin" , "Fourpenny piece" , "Old coin worth four pence" , "Old English silver coin" , "Old English coin worth four pennies" .)