20,21 a Sticky stuff , 21 down, doesn't make infernal paving .
Answer turns out to be "Good intentions" with good intentions being the road to hell.
However to get there you have to start with "goo" sticky stuff then add an anagram of the answer to 21d and doesn't.
The answer to 21d is "innit". Is it fair to need to have solved 21d before being able to work out the anagram fodder?