Old coin Rex found in butter! (5)
I believe the answer is:
groat
'old coin' is the definition.
(I've seen this before)
'rex found in butter' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'found in' is an insertion indicator.
'butter' becomes 'goat' (animal which butts).
'r' inserted within 'goat' is 'GROAT'.
(Other definitions for groat that I've seen before include "Old coin worth four pence" , "Fourpenny piece" , "Old English coin worth four pennies" , "Old English silver coin" , "Old four-pence coin" .)