A nine-ounce bloom developed rarely (4,2,1,4,4)
I believe the answer is:
once in a blue moon
'rarely' is the definition.
(I've seen this before)
'a nine-ounce bloom developed' is the wordplay.
'developed' indicates anagramming the letters.
'a'+'nine'+'ounce'+'bloom'='anineouncebloom'
'anineouncebloom' 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" , "very rarely" , "Hardly ever (4,2,1,4,4)" , "with extremely low frequency'" , "very occasionally" .)