Part of stage scenery support covering sink (8)
I believe the answer is:
backdrop
'part of stage scenery' is the definition.
'backdrop' can be an answer for 'scenery' (backdrop is a kind of scenery). I am unsure of the 'part of stage' bit.
'support covering sink' is the wordplay.
'support' becomes 'back' (to back is to support or endorse).
'covering' says to put letters next to each other (on top of in down clue).
'sink' becomes 'drop' (synonyms).
'back'+'drop'='BACKDROP'
Can you help me to learn more?
(Other definitions for backdrop that I've seen before include "Against which action is played out" , "Setting for a scene" , "Scenery hung at the rear of a stage" , "Wider view" , "Painted curtain at rear of stage" .)