Run off, grabbing last of champagne for a laugh (6)
I believe the answer is:
scream
'for a laugh' is the definition.
(scream is a kind of laugh)
'run off grabbing last of champagne' is the wordplay.
'run off' becomes 'scram'.
'grabbing' is an insertion indicator.
'last of' indicates one should take the final letters.
The last letter of 'champagne' is 'e'.
'scram' going around 'e' is 'SCREAM'.
(Other definitions for scream that I've seen before include "A shrill yell as from fright" , "Utter a piercing cry" , "Hilarious character" , "'Loud, high-pitched cry (6)'" , "Extremely funny entertainer" .)