Impervious academic eats nothing (5)
I believe the answer is:
proof
'impervious' is the definition.
('proof' can be similar in meaning to 'impervious')
'academic eats nothing' is the wordplay.
'academic' becomes 'prof'.
'eats' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'prof' enclosing 'o' is 'PROOF'.
(Other definitions for proof that I've seen before include "Exposition ending QED?" , "Render impervious (to)" , "may need amending" , "Testimony" , "whisky's strength?" .)