Correct impression that nothing occupies the academic (5)
I believe the answer is:
proof
'correct impression' is the definition.
'proof' can be an answer for 'impression' (proof is a kind of impression). I'm unsure of the 'correct' bit.
'nothing occupies the academic' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'occupies' indicates putting letters inside.
'the academic' becomes 'prof' (short for professor).
'o' placed inside 'prof' is 'PROOF'.
'that' is the link.
Can you help me to learn more?
(Other definitions for proof that I've seen before include "Body of evidence" , "Convincing evidence" , "Render impervious (to)" , "Mathematical argument" , "Draft copy of printed text" .)