In France, salmon is a source of fibre (4)
I believe the answer is:
flax
'source of fibre' is the definition.
(flax fibre can be made from flax stalks)
'in france salmon is a' is the wordplay.
'in' means one lot of letters goes inside another.
'france' becomes 'f'.
'salmon' becomes 'lx' (I am not sure about this - if you are sure you should believe this answer much more).
'f'+'lx'='flx'
'flx' placed around 'a' is 'FLAX'.
(Other definitions for flax that I've seen before include "Plant cropped for linen" , "leaves from it make cloth" , "Linen thread" , "Blue flowering plant cultivated for its seeds and fibre" , "Fibre from a plant with blue flowers, used to make linen" .)