Crease navy sheets (5)
I believe the answer is:
linen
'sheets' is the definition.
(I've seen this before)
'crease navy' is the wordplay.
'crease' becomes 'line' (synonyms).
'navy' becomes 'N' (common abbreviation eg in RN Royal Navy).
'line'+'n'='LINEN'
(Other definitions for linen that I've seen before include "Cloth woven flax fibres" , "Fabric, originally just spun flax" , "bedding" , "Material made from flax" , "More than one sheet" .)