Seat with a rich design (5)
I believe the answer is:
chair
'seat' is the definition.
(chair is a kind of seat)
'with a rich design' is the wordplay.
'with' indicates putting letters inside.
'design' is an anagram indicator.
'rich' with letters rearranged gives 'chir'.
'a' put within '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" .)