Like one vessel within another, they hold water (6)
I believe the answer is:
Here is my best explanation:
'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'.
(Other definitions for sewers that I've seen before include "drains", "Conduits for waste matter", "Waste pipes" and "Underground waste pipes".)