Laughs heartily (5)
I believe the answer is:
roars
'laughs heartily' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for roars that I've seen before include "Deep bellows" , "Bellows like a bull" , "Sounds like a lion" , "Bellows, hollers" , "Calls like lion" .)