Almost impervious to drink - it's a stern arrangement (5-9)

I believe the answer is:

water-resistant

I'm a little stuck... Click here to teach me more about this clue!

'almost impervious' is the definition.
I can't judge whether this definition defines the answer.

'drink it's a stern arrangement' is the wordplay.
'drink' becomes 'water' (I've seen this before).
'arrangement' indicates an anagram.
'its'+'a'+'stern'='itsastern'
'itsastern' with letters rearranged gives 'resistant'.
'water'+'resistant'='WATER-RESISTANT'

'to' acts as a link.

Can you help me to learn more?

(Other definitions for water-resistant that I've seen before include "Good in showers" , "Good at keeping out moisture" , "Impervious to moisture" .)

I've seen this clue in The Daily Express.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.