Demand payment for onslaught (6)
I believe the answer is:
charge
This clue is a double definition.
'demand payment' is the first definition.
(synonyms)
'onslaught' is the second definition.
(charge is a kind of onslaught)
'for' is the link.
(Other definitions for charge that I've seen before include "Accuse or rush forward" , "Accuse; load, fill" , "protection" , "Move quickly and violently" , "Amount to pay - attack" .)