Colour's maintained on huge screen (7)
I believe the answer is:
reredos
'screen' is the definition.
(a reredos is an ornamental screen)
'colour's maintained on huge' is the wordplay.
'colour' becomes 'red' (red is a kind of colour).
'maintained' means one lot of letters goes inside another (inserted letters get controlled or maintained).
'on' becomes 're' ('on' can be mean 'with reference to').
'huge' becomes 'os' (abbreviation for outsize).
're'+'os'='reos'
'red' inserted inside 'reos' is 'REREDOS'.
(Other definitions for reredos that I've seen before include "Painted screen behind an altar" , "Red rose could show a screen behind the altar" , "Hanging in church" , "Wall decoration behind altar" , "Fireback" .)