Clasp ring found in ancient tower (6)
I believe the answer is:
brooch
'clasp' is the definition.
(synonyms)
'ring found in ancient tower' is the wordplay.
'ring' becomes 'o'.
'found in' means one lot of letters goes inside another.
'ancient tower' becomes 'broch' (type of prehistoric tower).
'o' inserted within 'broch' is 'BROOCH'.
(Other definitions for brooch that I've seen before include "Ornament worn fastened to clothing" , "Ornament pinned to clothing" , "prehistoric tower" , "Jewellery item with pin" , "Adornment for dress" .)