Sienna set terribly unfeeling (9)
I believe the answer is:
insensate
'unfeeling' is the definition.
(I've seen this in another clue)
'sienna set terribly' is the wordplay.
'terribly' indicates an anagram.
'sienna'+'set'='siennaset'
'siennaset' is an anagram of 'INSENSATE'.
(Other definitions for insensate that I've seen before include "Inanimate, unfeeling" , "Saint seen to be unfeeling" , "Devoid of feeling and consciousness" , "Foolish" .)