Key interior design standard (9)
I believe the answer is:
criterion
'standard' is the definition.
('criterion' can be a synonym of 'standard')
'key interior design' is the wordplay.
'key' becomes 'c' (I've seen this in other clues).
'design' indicates anagramming the letters.
'interior' is an anagram of 'riterion'.
'c'+'riterion'='CRITERION'
(Other definitions for criterion that I've seen before include "Standard; model" , "Touchstone" , "Standard by which something can be judged" , "Judging standard" , "Benchmark - standard" .)