One rejected a knight's title? It's a bloomer (4)
I believe the answer is:
iris
'it's a bloomer' is the definition.
(I've seen this before)
'one rejected a knight's title?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'rejected' shows that the letters should be reversed in order.
'a knight's title?' becomes 'sir' (knights are addressed as 'sir').
'sir' backwards is 'ris'.
'i'+'ris'='IRIS'
(Other definitions for iris that I've seen before include "Female" , "Woman" , "Garden flower - part of eye" , "Camera diaphragm" , "pupil's governor" .)