It’s not certain the Inland Revenue will go up to heaven! (5)

I believe the answer is:

risky

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'it's not certain the inland revenue will go up' is the definition.
I can't judge whether this defines the answer.

'to heaven' is the wordplay.
'to' becomes 'ri' (I can't explain this - if you can you should believe this answer much more).
'heaven' becomes 'sky'.
'ri'+'sky'='RISKY'

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(Other definitions for risky that I've seen before include "Unsafe" , "Potentially dangerous" , "Treacherous" , "not playing safe" , "Chancy" .)

I've seen this clue in the Evening Standard.

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