Sort of lining used to be covering in bed (8)
I believe the answer is:
wainscot
'sort of lining' is the definition.
(type of wooden lining)
'used to be covering in bed' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'covering' is an insertion indicator.
'bed' becomes 'cot' (cot is a type of bed).
'was' placed around 'in' is 'wains'.
'wains'+'cot'='WAINSCOT'
(Other definitions for wainscot that I've seen before include "Panelling on lower part of interior walls" , "Wooden wall lining" , "Wood panelling on lower walls of room" , "panels" , "Lining of a wood-panelled wall" .)