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

The straight part
We know that "Hollow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hollow = dingle.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 DINGLE mean?
DINGLE noun- A small, narrow or enclosed, usually wooded valley.