Origin, root (6)
I believe the answer is:
source
This clue is a double definition.
'origin' is the first definition.
(I know that origin can be written as source)
'root' is the second definition.
(I know that root can be written as source)
(Other definitions for source that I've seen before include "Fountainhead - journalist's informant" , "'Origin, root (6)'" , "Where something originates" , "Origin; head of stream" , "Origin; spring" .)