Item of jewellery Oscar found in prehistoric tower (6)
I believe the answer is:
brooch
'item of jewellery' is the definition.
'oscar found in prehistoric tower' is the wordplay.
'oscar' becomes 'o' (phonetic alphabet: alpha, bravo, charlie etc.).
'found in' means one lot of letters goes inside another.
'prehistoric tower' becomes 'broch' (type of prehistoric structure found in Scotland).
'o' placed inside 'broch' is 'BROOCH'.
(Other definitions for brooch that I've seen before include "Ornament fastened with a pin" , "Ornamental hinged pin with clasp" , "Pinned jewellery item" , "Piece of jewellery" , "Breastpin" .)