The most common and most likely answer to this crossword clue is the 10 letter word BLOCKHOUSE
I've seen this crossword clue in The Times
B
L
O
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K
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Buddy explains!

The straight part
We know that "concrete shelter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case concrete shelter = blockhouse.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 BLOCKHOUSE mean?
BLOCKHOUSE noun- A sturdy military fortification, often of concrete, with gunports.
- A reinforced building from which to control hazardous operations, such as an explosion or a rocket launch.
- A temporary wooden fortification with a projecting upper story.