Mess tart about in bed (8)
I believe the answer is:
mattress
'in bed' is the definition.
('mattress' is part of 'bed')
'mess tart about' is the wordplay.
'about' indicates anagramming the letters.
'mess'+'tart'='messtart'
'messtart' with letters rearranged gives 'MATTRESS'.
(Other definitions for mattress that I've seen before include "Large thick pad on the bed" , "Sleeping support" , "Palliasse" , "Flat sleeping pad" , "Smartest kind of bed-base" .)