The most common and most likely answer to this crossword clue is the 6 letter word VILLAS
I've seen this crossword clue in The Guardian
V
I
L
L
A
S
More clues leading to the result VILLAS
Buddy explains!

The straight part
We know that "large country houses" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case large country houses = villas.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 VILLAS mean?
VILLAS noun- (plural "villas") A house, often larger and more expensive than average, in the countryside or on the coast, often used as a retreat.
- (plural "villas") A family house, often semi-detached, in a middle class street.
- (plural "villae") A country house, with farm buildings around a courtyard.