Fine and soft plant fibres (4)
I believe the answer is:
flax
'plant fibres' is the definition.
(fibre made from stalks of flax plants)
'fine and soft' is the wordplay.
'fine' becomes 'f' (abbreviation).
'and' means one lot of letters go next to another.
'soft' becomes 'lax' (synonyms).
'f'+'lax'='FLAX'
(Other definitions for flax that I've seen before include "Fibre woven to make linen" , "Plant cropped for linen" , "Source of linen" , "Plant from which linen is made" , "Linen thread" .)