Academic accepting nothing is watertight (5)
I believe the answer is:
proof
'watertight' is the definition.
(thesaurus)
'academic accepting nothing' is the wordplay.
'academic' becomes 'prof'.
'accepting' indicates putting letters inside.
'nothing' becomes 'o' (looks like zero - 0).
'prof' going around 'o' is 'PROOF'.
'is' is the link.
(Other definitions for proof that I've seen before include "Mathematical argument" , "Evidence that establishes the truth" , "to give a watertight case?" , "Body of evidence" , "Draft copy of printed text" .)