Chaotic detective's military assistant (10)
I believe the answer is:
disorderly
'chaotic' is the definition.
(I know that chaotic can be written as disorderly)
'detective's military assistant' is the wordplay.
'detective's' becomes 'di's' (Detective Inspector).
'military assistant' becomes 'orderly' (soldier carrying out tasks for another).
'dis'+'orderly'='DISORDERLY'
(Other definitions for disorderly that I've seen before include "a mess" , "Companion of drunk" , "Unruly" , "undisciplined" , "Lacking organisation" .)