Fork of a main stem (6)
I believe the answer is:
branch
'fork of a main stem' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for branch that I've seen before include "Divide (as a tree)" , "Part of tree/river" , "Bough - offshoot - secondary office" , "part of a big organisation" , "Lateral extension of river or railway" .)