The most common and most likely answer to this crossword clue is the 8 letter word THINNING
I've seen this crossword clue in The Daily Telegraph
T
H
I
N
N
I
N
G
More clues leading to the result THINNING
Buddy explains!

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 THINNING mean?
THINNING verb- To make thin or thinner.
- To become thin or thinner.
- To dilute.
- To remove some plants or parts of plants in order to improve the growth of what remains.
- Process of removing plants or parts of plants to facilitate the growth of other plants
- Wood, leaves, or other plant parts removed during the process of thinning
- The action or result of reducing thickness, density, or number
- Becoming less thick, dense, or numerous