Rushing water rushes maintaining pressure (6)
I believe the answer is:
rapids
'rushing water' is the definition.
'rushes maintaining pressure' is the wordplay.
'rushes' becomes 'raids' (both can mean an attack).
'maintaining' is an insertion indicator.
'pressure' becomes 'p' (abbreviation).
'raids' placed around 'p' is 'RAPIDS'.
(Other definitions for rapids that I've seen before include "Part of a river where the current is very fast" , "Fast-flowing river section" , "Where a river runs very fast" , "Parts of a river where the current is very fast" , "Fast river flows, white waters" .)