Remnants of one vessel within another (5)
I believe the answer is:
stubs
'remnants of one vessel within another' is the definition.
I can't judge whether this defines the answer.
'remnants of one vessel within' is the wordplay.
'remnants of one' becomes 'ss' (I can't explain this - if you can you should give a lot more credence to this answer).
'vessel' becomes 'tub' (tub is a kind of vessel).
'within' indicates putting letters inside.
'ss' enclosing 'tub' is 'STUBS'.
Can you help me to learn more?
(Other definitions for stubs that I've seen before include "residual pieces" , "Butts" .)