Plant from which linen is made (4)
I believe the answer is:
flax
'plant from which linen is made' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for flax that I've seen before include "Linen fibres" , "Textile fibre from a blue-flowered plant" , "Source of linen" , "Linen thread" , "Plant cropped for linen" .)