Prank with egg is bloomer (8)
I believe the answer is:
larkspur
'bloomer' is the definition.
(larkspur is a kind of flowering plant)
'prank with egg' is the wordplay.
'prank' becomes 'lark' (I've seen this before).
'with' means one lot of letters go next to another.
'egg' becomes 'spur' (I've seen this before).
'lark'+'spur'='LARKSPUR'
'is' acts as a link.
(Other definitions for larkspur that I've seen before include "Cultivated plant of the genus Delphinium" , "Flower (named after excrescence on a bird?)" , "Mediterranean plant of the buttercup family" , "Delphinium-like plant" , "Plant with flower spikes" .)