Part of a river quiet during incursions (6)
I believe the answer is:
rapids
'part of a river' is the definition.
(I know that A River depicts rapids)
'quiet during incursions' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'during' is an insertion indicator.
'incursions' becomes 'raids' (raid is a kind of incursion).
'p' put within 'raids' is 'RAPIDS'.
(Other definitions for rapids that I've seen before include "White water area" , "Part of a river where the current is very fast" , "Parts of a river where the current is very fast" , "Where a river runs very fast" , "Where rivers run fast" .)