Getting back, help clan in onslaught (8)
I believe the answer is:
diatribe
'onslaught' is the definition.
(I have seen 'Verbal onslaught' mean 'diatribe' so perhaps 'onslaught' could also mean 'diatribe')
'getting back help clan' is the wordplay.
'getting back' says the letters should be written backwards.
'help' becomes 'aid' ('aid' can be a synonym of 'help').
'clan' becomes 'tribe' ('tribe' can be a synonym of 'clan'**).
'aid' backwards is 'dia'.
'dia'+'tribe'='DIATRIBE'
'in' acts as a link.
(Other definitions for diatribe that I've seen before include "Long speech" , "Stream of invective" , "critical speech" , "Strong words" , "stream of abuse" .)