React to something hilarious (4)
I believe the answer is:
howl
'react to something hilarious' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for howl that I've seen before include "Bay - cry" , "Bay, clamour" , "Long cry of pain" , "Cry (with rage or laughter)" , "Long wailing cry" .)