Plain paper covering almost everything up (4)
I believe the answer is:
flat
'plain' is the definition.
(flat is a kind of plain)
'paper covering almost everything up' is the wordplay.
'paper' becomes 'ft'.
'covering' is an insertion indicator.
'almost' means to remove the last letter (most of the word but not all of it).
'everything' becomes 'all' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'all' with its final letter removed is 'al'.
'al' back-to-front is 'la'.
'ft' going around 'la' is 'FLAT'.
(Other definitions for flat that I've seen before include "rooms exactly" , "one or more in uniform" , "Horizontal" , "Quarters - punctured - smooth" , "Pressing implement" .)