Extraordinarily announce boom? Lie -- bust! (4,2,1,4,4)
I believe the answer is:
once in a blue moon
'extraordinarily' is the definition.
'announce boom? lie bust' is the wordplay.
'bust' indicates an anagram.
'announce'+'boom'+'lie'='announceboomlie'
'announceboomlie' is an anagram of 'ONCE IN A BLUE MOON'.
(Other definitions for once in a blue moon that I've seen before include "on very rare occasions" , "with extremely low frequency'" , "very occasionally" , "Hardly ever (4,2,1,4,4)" , "very rarely" .)