Excellent quip (5)
I believe the answer is:
crack
I believe this clue is a double definition.
'excellent' is the first definition.
(the most excellent, as in a 'crack team')
'quip' is the second definition.
(I know that quip can be written as crack)
(Other definitions for crack that I've seen before include "attempt" , "Break sharply; a joke" , "Opening; sharp blow" , "Witticism" , "Narrow opening, break" .)