This flower is a stupid mistake (7)
I believe the answer is:
bloomer
I believe this clue is a double definition.
'this flower' is the first definition.
(I've seen this before)
'stupid mistake' is the second definition.
(I've seen this before)
'is a' acts as a link.
(Other definitions for bloomer that I've seen before include "Big mistake" , "that's made with flour!" , "Large slip" , "Loaf of bread" , "rose, say" .)