The most common and most likely answer to this crossword clue is the 5 plus 3 plus 4 letter word BELOW THE BELT
B
E
L
O
W
T
H
E
B
E
L
T
More clues leading to the result BELOW THE BELT
- Not sporting pants here?
- Trouser pockets are dirty
- Having lost bottle, he blew unfairly
- Not just teeth wobble about when drinking litres
- Where flies are unfair
- Unworthy of goddess left in race when disregarding the rules
- Unfair punch: man, that hurts inside ring?
- He let wobble go wobbly ahead of time? That’s not fair!
- Where one sees drainpipes disregarding rules
- Unacceptable location of trouser pocket?
Buddy explains!

The straight part
We know that "Illegal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case illegal = below the belt.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.