Bounders impounding river ferries (2-3)
I believe the answer is:
ro-ros
'bounders' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'impounding river ferries' is the wordplay.
'impounding' is an insertion indicator.
'river' becomes 'r' (abbreviation).
'ferries' becomes 'roos' (I can't explain this - if you can you should believe this answer much more).
'r' going into 'roos' is 'RO-ROS'.
Can you help me to learn more?
(Another definition for ro-ros that I've seen is " Car ferries".)