Excited about nothing? That's a laugh! (4)
I believe the answer is:
hoot
'that's a laugh' is the definition.
(thesaurus)
'excited about nothing?' is the wordplay.
'excited' becomes 'hot' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'nothing?' becomes 'o' (looks like zero - 0).
'hot' enclosing 'o' is 'HOOT'.
(Other definitions for hoot that I've seen before include "scream" , "Honk - laugh" , "Owl's call" , "Owlish cry" , "what's voiced in parliament?" .)