Nothing must stop academic demonstration (5)
I believe the answer is:
proof
'demonstration' is the definition.
(I've seen this in another clue)
'nothing must stop academic' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'must stop' is an insertion indicator (stopping can mean blocking a gap).
'academic' becomes 'prof' (short for professor).
'o' going into 'prof' is 'PROOF'.
(Other definitions for proof that I've seen before include "paper for correcting" , "Body of evidence" , "Exposition ending QED?" , "Convincing evidence" , "a writer may check" .)