Nasty bit of GBH (8)
I believe the answer is:
grievous
'nasty' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for grievous that I've seen before include "Legal assault charge" , "This sort of bodily harms is serious" , "Deplorable" , "The "G" of GBH" , "Calamitous" .)