Master having overnight cases (6)
I believe the answer is:
govern
'master' is the definition.
(thesaurus)
'having overnight cases' is the wordplay.
'cases' says the answer is hidden in the clue.
'GOVERN' is hidden within 'having overnight'.
(Other definitions for govern that I've seen before include "Control and direct the affairs of" , "'Rule over, control (6)'" , "Exercise authority over a people" , "Run" , "Manage - control" .)