The most common and most likely answer to this crossword clue is the 6 letter word WADERS
I've seen this crossword clue in The Daily Telegraph
W
A
D
E
R
S
Buddy explains!

The straight part
We know that "Big boots" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case big boots = waders.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 WADERS mean?
WADERS noun- One who wades.
- (chiefly in the plural) A waterproof boot that comes up to the hip, used by fishermen, etc.
- A long-legged bird associated with wetland or coastal environments.