The most common and most likely answer to this crossword clue is the 8 letter word THINGAMY
T
H
I
N
G
A
M
Y
More clues leading to the result THINGAMY
Buddy explains!

The straight part
We know that "whatnot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whatnot = thingamy.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 THINGAMY mean?
THINGAMY noun- A thing (used in a vague way to refer to something whose name one cannot recall).
- Penis.
- A person whose name is either unknown or forgotten