The most common and most likely answer to this crossword clue is the 7 plus 6 letter word NORFOLK BROADS
I've seen this crossword clue in Financial Times
N
O
R
F
O
L
K
B
R
O
A
D
S
Buddy explains!
The straight part
We know that "lake district" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lake district = norfolk broads.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.