Hilda is upset over a bloomer (6)
I believe the answer is:
dahlia
'bloomer' is the definition.
(I have seen 'conspicuous bloomer' mean 'dahlia' so perhaps 'bloomer' could also mean 'dahlia')
'hilda is upset over a' is the wordplay.
'is upset' indicates an anagram.
'over' is an insertion indicator.
'hilda' is an anagram of 'dhlia'.
'dhlia' placed around 'a' is 'DAHLIA'.
(Other definitions for dahlia that I've seen before include "Wooster's aunt" , "Garden plant with large brightly-coloured flowers" , "Flower with pompom variety" , "one in bed" , "Plant of the daisy family" .)