Commissioner in mourning over northerners (8)
I believe the answer is:
governor
'commissioner' is the definition.
(both can mean a local official)
'in mourning over northerners' is the wordplay.
'in' says the answer is hidden in the clue.
'GOVERNOR' is hidden in the letters of 'mourning over northerners'.
(Other definitions for governor that I've seen before include "chief magistrate)" , "Device to control speed of engine" , "Official appointed to rule" , "School official" , "Viceroy" .)