Impression is a success (9)
I believe the answer is:
sensation
This clue is a double definition.
'impression' is the first definition.
(thesaurus)
'a success' is the second definition.
(thesaurus)
'is' acts as a link.
(Other definitions for sensation that I've seen before include "Feeling produced by stimulation, or dramatic news (9)" , "Feeling; thrilling person or thing" , "Feeling, awareness of stimulation" , "Scandal" , "Physical feeling or perception" .)