The most common and most likely answer to this crossword clue is the 7 letter word SANDPIT
I've seen this crossword clue in The Sunday Telegraph
S
A
N
D
P
I
T
More clues leading to the result SANDPIT
- Where castles built with double fences in
- Part of grounds and pitch where kids can play
- Sit around then by piano, in a place where youngsters can play
- Some courts and pitches where children may play
- Smooth Italian supporting quiet area for kids to play in
- Playing area in light fall of rain as it were?
- Where youngsters play could make you spit!
- Hospital department overlooking untidy place? Children play there
- Sit around and essentially respect this play place
- Letters to spell ‘spit’? That’s child’s play
Buddy explains!

The straight part
We know that "shallow box" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shallow box = sandpit.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 SANDPIT mean?
SANDPIT noun- A place or pit from which sand is excavated.
- A children’s play area consisting of a large container filled with sand.
- A small-scale illustrative model of the theater of war in the Middle East.