Meeting back in Illinois’s essential (7)
I believe the answer is:
'meeting' is the definition.
(I know that session is a type of meeting)
'back in illinois's essential' is the wordplay.
'back' is a reversal indicator.
'in' indicates the answer is hidden within the clue.
'noisses' is hidden within 'illinoiss essential'.
'noisses' backwards is 'SESSION'.
(Other definitions for session that I've seen before include "School term" , "Period spent on a single activity" , "Period devoted to an activity" , "Hearing, sitting" , "Studio recording period" .)