Academic must collect old evidence (5)
I believe the answer is:
proof
'evidence' is the definition.
(evidence proves something)
'academic must collect old' is the wordplay.
'academic' becomes 'prof' (short for professor).
'must collect' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'prof' placed around 'o' is 'PROOF'.
(Other definitions for proof that I've seen before include "Test" , "Trial printing" , "Decisive evidence" , "Author may have to check this" , "Testimony" .)