Exciting? Alas, one isn’t excited (11)
I believe the answer is:
sensational
'exciting?' is the definition.
(thesaurus)
'alas one isn't excited' is the wordplay.
'excited' indicates an anagram.
'alas'+'one'+'isnt'='alasoneisnt'
'alasoneisnt' with letters rearranged gives 'SENSATIONAL'.
(Other definitions for sensational that I've seen before include "Excellent" , "Fantastic" , "Is feeling so" , "Staggering" , "Slain so neat (anag) -- drop-dead" .)