Moulding, very old, mostly seen in castles (5)
I believe the answer is:
ovolo
'moulding' is the definition.
(I know that ovolo is a type of moulding)
'very old mostly seen in castles' is the wordplay.
'very' becomes 'v' (abbreviation).
'mostly seen' means to remove the last letter.
'in' means one lot of letters goes inside another.
'castles' becomes 'oo' (I can't explain this - if you can you should give a lot more credence to this answer).
'v'+'old'='vold'
'vold' with its final letter taken away is 'vol'.
'vol' placed within 'oo' is 'OVOLO'.
Can you help me to learn more?
(Other definitions for ovolo that I've seen before include "decorative strip" , "Thumb" , "Convex moulding" , "Rounded architectural moulding" .)