Extremely rare, brightly coloured, very large screen (7)
I believe the answer is:
reredos
'screen' is the definition.
(a reredos is an ornamental screen)
'extremely rare brightly coloured very large' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'brightly coloured' becomes 'red' (red is often a bright colour).
'very large' becomes 'os' (abbreviation for oversize).
'rare' with its centre taken out is 're'.
're'+'red'+'os'='REREDOS'
(Other definitions for reredos that I've seen before include "Fireback" , "Red rose could show a screen behind the altar" , "Back of fireplace" , "Painted screen behind an altar" , "Wall decoration behind altar" .)