Like one vessel within another, they hold water (6)
I believe the answer is:
sewers
'another they hold water' is the definition.
The definition suggests an adverb but the answer is not.
'like one vessel within' is the wordplay.
'like one' becomes 'ss' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'vessel' becomes 'ewer' (ewer is a kind of vessel).
'within' means one lot of letters goes inside another.
'ss' enclosing 'ewer' is 'SEWERS'.
Can you help me to learn more?
(Other definitions for sewers that I've seen before include "drains" , "Conduits for waste matter" , "Underground waste pipes" .)