Nothing will stop sweet academic, that’s certain (9)
I believe the answer is:
foolproof
'certain' is the definition.
(synonyms)
'nothing will stop sweet academic' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'will stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'sweet' becomes 'fool' (type of dessert or sweet).
'academic' becomes 'prof' (short for professor).
'fool'+'prof'='foolprof'
'o' inserted within 'foolprof' is 'FOOLPROOF'.
'that's' acts as a link.
(Other definitions for foolproof that I've seen before include "Can't be misinterpreted, even by simpleton" , "Not admitting failure" , "Completely safe" , "sure to work" , "Not liable to failure" .)