Crossword Genius

Lacking the capacity to host for five? Impossible to justify (10)

Ross

I believe the answer is:

unprovable

'impossible to justify' is the definition.

'lacking the capacity to host for five?' is the wordplay.
'lacking the capacity' becomes 'unable'.
'to host' is an insertion indicator.
'for' becomes 'pro' (I've seen this before).
'five?' becomes 'v' ('V' can be a synonym of 'five').
'pro'+'v'='prov'
'unable' enclosing 'prov' is 'UNPROVABLE'.

I've seen this clue in The Telegraph.
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