Seat with a rich design (5)
I believe the answer is:
chair
'seat' is the definition.
(I know that seat can be written as chair)
'with a rich design' is the wordplay.
'with' means one lot of letters goes inside another.
'design' indicates anagramming the letters.
'rich' anagrammed gives 'chir'.
'a' put into 'chir' is 'CHAIR'.
(Other definitions for chair that I've seen before include "Item of furniture (which can be addressed!)" , "Prof's post; meeting leader" , "academic office" , "Professorship" , "A committee's most important seat" .)