Interior design involving some wide cornices (5)
I believe the answer is:
decor
'interior design' is the definition.
(I've seen this before)
'some wide cornices' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'DECOR' is hidden in the letters of 'wide cornices'.
'involving' acts as a link.
(Other definitions for decor that I've seen before include "Internal ornamentation" , "Colour-scheme etc of a room" , "(Room) design" , "ornament" , "Interior layout and furnishings" .)