The most common and most likely answer to this crossword clue is the 6 letter word WELLIE
I've seen this crossword clue in The Guardian
W
E
L
L
I
E
More clues leading to the result WELLIE
- Boot that is put on properly
- Boot that is fine at first
- 23 24 across in a good way, that is
- Turned poorly, squeezed into small footwear
- Vigour of compilers primarily liking deception
- It’s 50:50 one will fit into small boot
- Boot that is at the bottom of a hole in the ground
- Clearly that is a measure of effort?
- Boot that is following fit
- Boot that is holding up properly
Buddy explains!

The straight part
We know that "boot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case boot = wellie.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 WELLIE mean?
WELLIE noun- Wellington boot.
- Force on a pedal or increase to any fuel or power for an engine or motor.
- Force or effort.