Bloomer concealed by old Roman Catholic (6)
I believe the answer is:
orchid
'bloomer' is the definition.
(orchid is a kind of flowering plant)
'concealed by old roman catholic' is the wordplay.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'by' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'roman catholic' becomes 'rc' (abbreviation for Roman Catholic).
'o'+'rc'='orc'
'hid' after 'orc' is 'ORCHID'.
(Other definitions for orchid that I've seen before include "Often prized flowering plant" , "Plant with showy flowers" , "Chord I turn into a flower" , "Exotic flower with bee and lady varieties" , "Family of flowers with unusual shapes and beautiful colours" .)