Hateful big gun meeting half-hearted mob (9)
I believe the answer is:
execrable
'hateful' is the definition.
(similar in meaning)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for execrable that I've seen before include "Abominable, deserves to be detested" , "Abominable, detestable" , "Accursed" , "Appallingly bad" , "Hated" .)