The most common and most likely answer to this crossword clue is the 5 plus 5 letter word GRAVY TRAIN
I've seen this crossword clue in The Daily Telegraph
G
R
A
V
Y
T
R
A
I
N
More clues leading to the result GRAVY TRAIN
Buddy explains!

The straight part
We know that "Source of easy money" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source of easy money = gravy train.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.