Opening joke (5)
I believe the answer is:
crack
I believe this is a double definition.
'opening' is the first definition.
(I know that crack is a type of opening)
'joke' is the second definition.
(a crack is a joke or quip)
(Other definitions for crack that I've seen before include "Brilliant" , "Narrow opening, break" , "Go for expert" , "Break sharply; a joke" , "Take severe measures - . . . . . down on" .)