Unusual credo of interior design (5)
I believe the answer is:
decor
'interior design' is the definition.
(I've seen this before)
'unusual credo' is the wordplay.
'unusual' indicates an anagram.
'credo' anagrammed gives 'DECOR'.
'of' is the link.
(Other definitions for decor that I've seen before include "Colour scheme, furnishing etc. of a room" , "Embellishment of a room" , "Scheme of interior furnishings" , "Credo (anag.)" , "Choice of colour" .)