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 judge whether it can be defined by this definition.
'impounding river ferries' is the wordplay.
'impounding' means one lot of letters goes inside another.
'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".)