Sack's covering penny or something in river (6)
I believe the answer is:
rapids
'something in river' is the definition.
I can't tell whether this definition defines the answer.
'sack's covering penny' is the wordplay.
'sack's' becomes 'raids' (I've seen this in other clues).
'covering' means one lot of letters goes inside another.
'penny' becomes 'p'.
'raids' enclosing 'p' is 'RAPIDS'.
'or' is the link.
Can you help me to learn more?
(Other definitions for rapids that I've seen before include "Where rivers run fast" , "Fast-flowing river stretches, white water" , "fast flowing water" , "Part of a river where the current is very fast" , "Fast-flowing river section" .)