The most common and most likely answer to this crossword clue is the 7 plus 7 letter word THOUGHT PROCESS
I've seen this crossword clue in The Sunday Telegraph
T
H
O
U
G
H
T
P
R
O
C
E
S
S
Buddy explains!
The straight part
We know that "source for idea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source for idea = thought process.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.