Killer at sea concealed bloomer (6)
I believe the answer is:
orchid
'bloomer' is the definition.
(orchid is a kind of flowering plant)
'killer at sea concealed' is the wordplay.
'killer at sea' becomes 'orc' (historic term for a sea creature).
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'orc'+'hid'='ORCHID'
(Other definitions for orchid that I've seen before include "elaborate bloomer" , "Often prized flowering plant" , "Wild (or much cultivated) flower" , "Exotic flower with bee and lady varieties" , "Flowering plant with bee and lady varieties" .)