The most common and most likely answer to this crossword clue is the 8 letter word AWAITING
I've seen this crossword clue in The Guardian
A
W
A
I
T
I
N
G
More clues leading to the result AWAITING
Buddy explains!

The straight part
We know that "in store" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in store = awaiting.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 AWAITING mean?
AWAITING verb- To wait for.
- To expect.
- To be in store for; to be ready or in waiting for.
- To serve or attend; to wait on, wait upon.
- To watch, observe.
- To wait; to stay in waiting.