Fine loose fibre (4)
I believe the answer is:
flax
'fibre' is the definition.
(I know that flax is a type of plant fiber)
'fine loose' is the wordplay.
'fine' becomes 'f' (abbreviation).
'loose' becomes 'lax' (associated in meaning).
'f'+'lax'='FLAX'
(Other definitions for flax that I've seen before include "Fibre from a plant with blue flowers, used to make linen" , "Linen thread" , "Blue flowering plant cultivated for its seeds and fibre" , "Linen fibres" , "Plant yielding linen" .)