Dealt with gunshot outside Los Angeles in fierce attack (9)
I believe the answer is:
onslaught
'fierce attack' is the definition.
(I've seen this before)
'dealt with gunshot outside los angeles' is the wordplay.
'dealt with' indicates an anagram.
'outside' is an insertion indicator.
'los angeles' becomes 'la'.
'gunshot' anagrammed gives 'onsught'.
'onsught' going around 'la' is 'ONSLAUGHT'.
'in' is the link.
(Other definitions for onslaught that I've seen before include "Vicious attack" , "Violent attack" , "Heavy attack" , "Assault" , "Sudden and severe onset of trouble" .)