Excellent witticism in front of guests (8)
I believe the answer is:
cracking
'excellent' is the definition.
(cracking can mean excellent or brilliant)
'witticism in front of guests' is the wordplay.
'witticism' becomes 'crack' (I've seen this in another clue).
'front of' suggests taking the first letters (the letter at the front of the word).
The first letter of 'guests' is 'g'.
'crack'+'in'+'g'='CRACKING'
(Other definitions for cracking that I've seen before include "failing" , "very fast" , "Excellent; fracturing" , "Solving" , "Wonderful" .)