The most common and most likely answer to this crossword clue is the 6 letter word EXPEND
I've seen this crossword clue in The Times
E
X
P
E
N
D
More clues leading to the result EXPEND
Buddy explains!

The straight part
We know that "Lavish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lavish = expend.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 EXPEND mean?
EXPEND verb- To consume, exhaust (some resource)
- (of money) to spend, disburse