Academic suppresses old evidence (5)
I believe the answer is:
proof
'evidence' is the definition.
(proof is a kind of evidence)
'academic suppresses old' is the wordplay.
'academic' becomes 'prof'.
'suppresses' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'prof' going around 'o' is 'PROOF'.
(Other definitions for proof that I've seen before include "Render impervious (to)" , "Demonstration of the truth of something" , "mathematical tour de force" , "may need amending" , "Strength of spirit" .)