Source of fibres, fine and loose (4)
I believe the answer is:
flax
'source of fibres' is the definition.
'flax' can be an answer for 'source' (I have seen 'Source of linseed' mean 'flax' so perhaps 'source' could also mean 'flax'). I am not sure about the 'of fibres' bit.
'fine and loose' is the wordplay.
'fine' becomes 'f' (abbreviation).
'and' means one lot of letters go next to another.
'loose' becomes 'lax' (synonyms).
'f'+'lax'='FLAX'
Can you help me to learn more?
(Other definitions for flax that I've seen before include "Plant yielding linen" , "Plant cropped for linen" , "Plant from which linen is made" , "Fibre woven to make linen" , "Blue flowering plant cultivated for its seeds and fibre" .)