Excellent joke (5)
I believe this is a double definition.
'excellent' is the first definition.
(the most excellent, as in a 'crack team')
'joke' is the second definition.
(a crack is a joke or quip)
(Other definitions for crack that I've seen before include "First-rate" , "Go for expert" , "Take severe measures - . . . . . down on" , "Break sharply; a joke" , "Narrow opening, break" .)
(Other definitions for corker that I've seen before include "One bottling wine" , "Special one" , "Bottle-stopper for excellent person" , "beauty" , "smashing one" .)