The most common and most likely answer to this crossword clue is the 6 letter word THENCE
I've seen this crossword clue in The Times
T
H
E
N
C
E
Buddy explains!

The straight part
We know that "from such a source" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case from such a source = thence.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does THENCE mean?
THENCE adverb- From there, from that place or from that time.
- Deriving from this fact or circumstance; therefore, therefrom.
- From that time; thenceforth; thereafter