Crease? Any number in sheets, etc (5)
I believe the answer is:
linen
'sheets etc' is the definition.
(bed sheets might be made of linen)
'crease? any number' is the wordplay.
'crease?' becomes 'line' ('line' can be a synonym of 'crease').
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'line'+'n'='LINEN'
'in' is the link.
(Other definitions for linen that I've seen before include "Flaxen fabric" , "sheets, perhaps" , "Cloth made from flax" , "Holland, say" , "Flax product" .)