Source of fibre that is served with butter (5)
I believe the answer is:
ramie
'source of fibre' is the definition.
(fibrous vegetable)
'that is served with butter' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'served with' means one lot of letters go next to another.
'butter' becomes 'ram' (a ram is something which 'butts').
'ie' put after 'ram' is 'RAMIE'.
(Other definitions for ramie that I've seen before include "plant" , "Tough fibre" .)