The most common and most likely answer to this crossword clue is the 7 letter word SANDBOX
I've seen this crossword clue in The Daily Telegraph
S
A
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Buddy explains!
The straight part
We know that "It’s full of grains" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s full of grains = sandbox.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 SANDBOX mean?
SANDBOX noun- A children's play area consisting of a box filled with sand.
- A box filled with sand that is shaped to form a mould for metal casting.
- A container for sand or pounce, used historically before blotting paper.
- An animal's litter box.
- A box carried on locomotives, from which sand runs onto the rails in front of the driving wheels, to prevent slipping.
- An isolated area where a program can be executed with a restricted portion of the resources available.
- (Wiktionary and WMF jargon) A page on a wiki where users are free to experiment without destroying or damaging any legitimate content.
- (usually "The Sandbox") The Middle East.
- To restrict (a program, etc.) by placing it in a sandbox.