The most common and most likely answer to this crossword clue is the 5 letter word WELLY
I've seen this crossword clue in Financial Times
W
E
L
L
Y
More clues leading to the result WELLY
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 = welly.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.
Crossword Answer definitions
WELLY noun- Wellington boot.
- Force on a pedal or increase to any fuel or power for an engine or motor.
- Force or effort.
- Almost; nearly.