The most common and most likely answer to this crossword clue is the 8 letter word DIPSTICK
I've seen this crossword clue in The Guardian
D
I
P
S
T
I
C
K
More clues leading to the result DIPSTICK
- Idiot that tells you oil is running out?
- Fool takes the plunge a short time later
- Swim poorly, swallowing tons — idiot!
- Dummy declines HP
- Jerk to go down, don’t twist!
- Wally, the pickpocket’s son, gets credit
- A measure of a pickpocket’s credit
- Pickpocket’s account of a fool
- Rodney say, giving guacamole and hummus the thumbs-up?
- Fool takes plunge short time later
Buddy explains!

The straight part
We know that "Wally" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wally = dipstick.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 DIPSTICK mean?
DIPSTICK noun- A stick or rod used to measure the depth of a liquid. Often used to check the level at which a liquid in an opaque or inaccessible tank or reservoir stands; gauge.
- A penis.
- A useless person of inferior intellect; a dipshit.
- To check (a person) for their understanding of something, as of a lesson, or attitude toward something.