By necessity some wallpaper for ceiling (8)
I believe the answer is:
perforce
'by necessity' is the definition.
(I've seen this before)
'some wallpaper for ceiling' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'PERFORCE' is hidden in the letters of 'wallpaper for ceiling'.
(Other definitions for perforce that I've seen before include "Unavoidably" , "Old word for necessarily, due to circumstances" , "willy-nilly" , "Necessarily, arising from free crop" , "By force of circumstance, necessarily" .)