Forming an architectural feature (8)
I believe the answer is:
moulding
I believe this is a double definition.
'forming' is the first definition.
(I know that moulding is a type of forming)
'an architectural feature' is the second definition.
(I know that moulding is a type of architectural element)
(Other definitions for moulding that I've seen before include "Decorative architectural feature" , "Gum on lid (anag)" , "going off?" , "Fillet" .)