The most common and most likely answer to this crossword clue is the 7 letter word POACHED
I've seen this crossword clue in The Daily Telegraph
P
O
A
C
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More clues leading to the result POACHED
Buddy explains!

The straight part
We know that "taken illegally" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case taken illegally = poached.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 POACHED mean?
POACHED verb- To cook something in simmering liquid.
- To be cooked in simmering liquid
- To become soft or muddy.
- To make soft or muddy.
- To stab; to pierce; to spear, as fish.
- To force, drive, or plunge into anything.
- To begin and not complete.
- To take game or fish illegally.
- To take anything illegally or unfairly.
- To entice (an employee or customer) to switch from a competing company to one's own.
- Cooked by poaching.
- Obtained by poaching.