Screen of ochre, red ostensibly? (7)
I believe the answer is:
reredos
'screen' is the definition.
(a reredos is an ornamental screen)
'of ochre red ostensibly?' is the wordplay.
'of' indicates a hidden word.
'REREDOS' is hidden in the letters of 'ochre red ostensibly'.
(Other definitions for reredos that I've seen before include "Ornamental screen behind a church altar" , "Back of fireplace" , "Church feature" , "Wall decoration behind altar" , "Hanging in church" .)