The most common and most likely answer to this crossword clue is the 12 letter word ONOMATOPOEIA
I've seen this crossword clue in The Guardian
O
N
O
M
A
T
O
P
O
E
I
A
More clues leading to the result ONOMATOPOEIA
Buddy explains!

The straight part
We know that "Thud may be/Thud may be" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thud may be/thud may be = onomatopoeia.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 ONOMATOPOEIA mean?
ONOMATOPOEIA noun- The property of a word of sounding like what it represents.
- A word that sounds like what it represents, such as "gurgle" or "hiss".
- The use of language whose sound imitates that which it names.