Sickness starts to nark all university students entering academy (6)
I believe the answer is:
nausea
'sickness' is the definition.
('nausea' can be a synonym of 'sickness')
'starts to nark all university students entering academy' is the wordplay.
'starts to' says to take the initial letters.
The first letters of 'nark all university students entering academy' is 'NAUSEA'.
(Other definitions for nausea that I've seen before include "Condition of feeling imminent sickness" , "Feeling of sickness or disgust" , "Malady" , "A feeling of disgust or illness" , "Feeling of revulsion" .)