Story involving knight? Mortification, ultimately, for the cloth (5)
I believe the answer is:
linen
'the cloth' is the definition.
(linen is a kind of cloth)
'story involving knight? mortification ultimately' is the wordplay.
'story' becomes 'lie' (story can mean a fabrication).
'involving' means one lot of letters goes inside another.
'knight?' becomes 'n' (chess).
'ultimately' indicates one should take the final letters.
The final letter of 'mortification' is 'n'.
'lie' enclosing 'n' is 'line'.
'line'+'n'='LINEN'
'for' acts as a link.
(Other definitions for linen that I've seen before include "clothes and sheets" , "Holland, say" , "Fabric from the fibres of the flax plant" , "Flax textile" , "Underclothes" .)