Illinois sergeant retires, concealing departure (9)

I believe the answer is:

egression

'departure' is the definition.
(thesaurus)

'illinois sergeant retires concealing' is the wordplay.
'concealing' says the answer is hidden in the clue.
'retires' says the letters should be written backwards.
'noisserge' is hidden within 'illinois sergeant'.
'noisserge' back-to-front is 'EGRESSION'.

(Another definition for egression that I've seen is " Emergence".)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app