Help over ethnic group in critical attack (8)
I believe the answer is:
diatribe
'critical attack' is the definition.
(a diatribe is an angry attack on someone)
'help over ethnic group' is the wordplay.
'help' becomes 'aid' (synonyms).
'over' is a reversal indicator.
'ethnic group' becomes 'tribe' (I've seen this before).
'aid' backwards is 'dia'.
'dia'+'tribe'='DIATRIBE'
'in' is the link.
(Other definitions for diatribe that I've seen before include "Long speech" , "I debar it, being a violent attack in words" , "Vehement criticism" , "Philippic" , "Strong words" .)