Gathering is a hit (4)
I believe the answer is:
bash
'is a hit' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for bash that I've seen before include "Party; beat" , "Try - rave-up" , "Hit hard; party (slang)" , "Shindig, function" , "Strike hard; party" .)