The most common and most likely answer to this crossword clue is the 6 letter word WINDOW
I've seen this crossword clue in The Sunday Telegraph
W
I
N
D
O
W
More clues leading to the result WINDOW
- Source of light twist that hurt!
- Dowager welcomes knight’s opening
- Opportunity to take cash prize on the radio
- Opening twist followed by cry of pain
- Opening profit on NYSE?
- Person looking for opportunities with skipper?
- One has no intention of buying computer system with large storage facility
- Environment with kangaroo, one that just looks
- Get a glimpse of what’s in store
- Viewer keen to see what’s in store
Buddy explains!

The straight part
We know that "Outsider who can see what’s in store" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case outsider who can see what’s in store = window.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 WINDOW mean?
WINDOW noun- The inedible parts of a grain-producing plant.
- Straw or hay cut up fine for the food of cattle.
- Any excess or unwanted material, resource, or person; anything worthless.
- Light jesting talk; banter; raillery.
- Loose material, e.g. small strips of aluminum foil dropped from aircraft, intended to interfere with radar detection.
- An opening, usually covered by one or more panes of clear glass, to allow light and air from outside to enter a building or vehicle.
- An opening, usually covered by glass, in a shop which allows people to view the shop and its products from outside; a shop window.
- The shutter, casement, sash with its fittings, or other framework, which closes a window opening.
- A period of time when something is available.
- A restricted range.
- A rectangular area on a computer terminal or screen containing some kind of user interface, displaying the output of and allowing input for one of a number of simultaneously running computer processes.
- A figure formed of lines crossing each other.
- The time between first infection and detectability.
- To furnish with windows.
- To place at or in a window.