The most common and most likely answer to this crossword clue is the 14 letter word COUNTERFEITING
I've seen this crossword clue in The Sunday Telegraph
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Buddy explains!
The straight part
We know that "making money illegally" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case making money illegally = counterfeiting.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 COUNTERFEITING mean?
COUNTERFEITING verb- To falsely produce what appears to be official or valid; to produce a forged copy of.
- To produce a faithful copy of.
- To feign; to mimic.
- (usually "be counterfeited") Of a turn or river card, to invalidate a player's hand by making a better hand on the board.
- The act of one who counterfeits.