Commotion starts in showers, then inmates riot (4)
I believe the answer is:
Here is my best explanation:
'commotion' is the definition.
(stir is a kind of commotion)
'starts in showers then inmates riot' is the subsidiary indication.
'starts in' suggests taking the first letters.
The initial letters of 'showers then inmates riot' is 'stir'.
(Other definitions for stir that I've seen before include "Mix or agitate", "Disturb", "Agitation - prison", "Whisk or rotate", "Move around - prison", "Prison ? commotion", "Arouse; mix", "A slangy prison - whisk it", "Excite emotionally" and "Prison (colloq.)".)