Linen source (4)
I believe the answer is:
flax
'linen source' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for flax that I've seen before include "Linen thread" , "Blue flowering plant cultivated for its seeds and fibre" , "leaves from it make cloth" , "Fibre woven to make linen" , "Fibre from a plant with blue flowers, used to make linen" .)