One bloom came up (5)
I believe the answer is:
arose
'came up' is the definition.
(I've seen this before)
'one bloom' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'bloom' becomes 'rose' (bloom can mean a flower).
'a'+'rose'='AROSE'
(Other definitions for arose that I've seen before include "Occurred; stood up" , "Followed" , "Came about" , "Happened" , "Became apparent" .)