Countless sources of useful news were informative (6)
I believe the answer is:
untold
'countless' is the definition.
(I know that countless can be written as untold)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for untold that I've seen before include "missing relations" , "Too many to be counted; not narrated" , "Not narrated or measured" , "What's indescribable" , "Incalculable amount" .)