Mark new sheets and pillow cases (5)
I believe the answer is:
linen
'sheets and pillow cases' is the definition.
(I've seen this in another clue)
'mark new' is the wordplay.
'mark' becomes 'line' (line is a kind of mark).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'line'+'n'='LINEN'
(Other definitions for linen that I've seen before include "Flaxen fabric" , "Cloth woven flax fibres" , "Underclothing" , "Fabric, originally just spun flax" , "fabricated? This is" .)