The most common and most likely answer to this crossword clue is the 9 letter word PERMEABLE
I've seen this crossword clue in The Guardian
P
E
R
M
E
A
B
L
E
Buddy explains!
The straight part
We know that "letting things through / letting things through" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case letting things through / letting things through = permeable.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 PERMEABLE mean?
PERMEABLE adjective- That absorbs or allows the passage of fluids