Cryptic crossword clue
Lacking the capacity to host for five? Impossible to justify 10

The most common and most likely answer to this crossword clue is the 10 letter word UNPROVABLE

I've seen this crossword clue in The Daily Telegraph
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Buddy explains!

The straight part
We know that "Impossible to justify" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case impossible to justify = unprovable.
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 UNPROVABLE mean?

UNPROVABLE adjective
  1. That cannot be proved or verified by any test

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