Keeps a ship out of a certain river (5)
I believe the answer is:
stays
'keeps' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Maybe you can see a link between them that I can't see?
'a ship out of a certain river' is the wordplay.
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'out of' means one lot of letters goes inside another (some letters on the outside of others).
'a certain river' becomes 'tay' (River Tay. I am not sure about the 'certain' bit.).
'ss' placed around 'tay' is 'STAYS'.
Can you help me to learn more?
(Other definitions for stays that I've seen before include "Supports" , "Settles" , "Remains; support wires" , "Stanchions" , "for a slimmer figure" .)