The most common and most likely answer to this crossword clue is the 6 letter word INCURS
I've seen this crossword clue in Independent
I
N
C
U
R
S
Buddy explains!

The straight part
We know that "Becomes liable for" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case becomes liable for = incurs.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 INCURS mean?
INCURS verb- To bring upon oneself or expose oneself to, especially something inconvenient, harmful, or onerous; to become liable or subject to
- To enter or pass into
- To fall within a period or scope; to occur; to run into danger