Is it smoking jacket that becomes one? (6)
I believe the answer is:
blazer
This clue is a double definition.
'is it smoking' is the first definition.
(something blazing or smoking)
'jacket that becomes one?' is the second definition.
(fancy jacket)
(Other definitions for blazer that I've seen before include "coat?" , "Loose-fitting jacket for school or team" , "School jacket (on fire?)" , "Light single-breasted (school or club) jacket" , "Man's garment" .)