Crack up as emu runs riot (5)
I believe the answer is:
amuse
'crack up' is the definition.
(I've seen this in another clue)
'as emu runs riot' is the wordplay.
'runs riot' is an anagram indicator (I've seen 'run riot' mean this (I've seen this in other clues)).
'as'+'emu'='asemu'
'asemu' anagrammed gives 'AMUSE'.
(Other definitions for amuse that I've seen before include "interest" , "Delight" , "Recreate" , "Keep occupied" , "Beguile" .)