Prepared rose-red altar screen (7)
I believe the answer is:
reredos
'altar screen' is the definition.
(I've seen this before)
'prepared rose-red' is the wordplay.
'prepared' is an anagram indicator.
'rose'+'red'='rosered'
'rosered' with letters rearranged gives 'REREDOS'.
(Other definitions for reredos that I've seen before include "Church feature" , "Painted screen behind an altar" , "Hanging in church" , "Ornamental screen behind a church altar" , "Red rose could show a screen behind the altar" .)