Unfortunate lapse is a bit of a bloomer (5)
I believe the answer is:
sepal
'a bit of a bloomer' is the definition.
(part of a flower)
'unfortunate lapse' is the wordplay.
'unfortunate' indicates an anagram.
'lapse' with letters rearranged gives 'SEPAL'.
'is' acts as a link.
(Other definitions for sepal that I've seen before include "part of leaf" , "Lapse about part of a flower" , "Part of the calyx of a flower" , "Violet" , "flowery bit" .)