The most common and most likely answer to this crossword clue is the 10 plus 7 letter word SINGLE-USE PLASTIC
I've seen this crossword clue in Financial Times
S
I
N
G
L
E
-
U
S
E
P
L
A
S
T
I
C
Buddy explains!
The straight part
We know that "our grandkids’ll pay for it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case our grandkids’ll pay for it = single-use plastic.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.