The most common and most likely answer to this crossword clue is the 6 plus 8 letter word ESTATE PLANNING
I've seen this crossword clue in Financial Times
E
S
T
A
T
E
P
L
A
N
N
I
N
G
Buddy explains!
The straight part
We know that "Lancelot Brown’s expertise/tax avoidance?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lancelot brown’s expertise/tax avoidance? = estate planning.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.