It's not certain the Inland Revenue will go up to heaven! (5)
I believe the answer is:
risky
'it's not certain the inland revenue will' is the definition.
I can't tell whether this definition defines the answer.
'up to heaven' is the wordplay.
'up to' becomes 'ri' (I can't explain this - if you can you should give a lot more credence to this answer).
'heaven' becomes 'sky' (I've seen this in another clue).
'ri'+'sky'='RISKY'
'go' acts as a link.
Can you help me to learn more?
(Other definitions for risky that I've seen before include "Unsafe" , "Potentially dangerous" , "Treacherous" , "not playing safe" , "Chancy" .)