Source, root (6)
I believe the answer is:
origin
I believe this clue is a double definition.
'source' is the first definition.
(I know that source can be written as origin)
'root' is the second definition.
(I know that root can be written as origin)
(Other definitions for origin that I've seen before include "Where X meets Y" , "Beginning, starting point" , "Crossing of axes" , "Source, derivation" , "Square one" .)