The most common and most likely answer to this crossword clue is the 12 letter word WAREHOUSEMEN
I've seen this crossword clue in The Times
W
A
R
E
H
O
U
S
E
M
E
N
Buddy explains!
The straight part
We know that "Chaps who should know what's in store" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chaps who should know what's in store = warehousemen.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 WAREHOUSEMEN mean?
WAREHOUSEMEN noun- A person who manages, or works in, a warehouse.
- One who keeps a wholesale shop for woollen goods.