For the third time, decorate small screen? (7)
I believe the answer is:
reredos
'screen?' is the definition.
(a reredos is an ornamental screen)
'for the third time decorate small' is the wordplay.
I cannot quite see how this works, but
'decorate' could be 'redo' (I've seen this in another clue) and 'redo' is found within the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
'the third' could be 'e' (3rd letter of 'the') and 'e' is present in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for reredos that I've seen before include "Wall decoration behind altar" , "Church feature" , "Fireback" , "Hanging in church" , "Red rose could show a screen behind the altar" .)