Plumb reliable (5)
I believe the answer is:
sound
I believe this is a double definition.
'plumb' is the first definition.
(thesaurus)
'reliable' is the second definition.
(I've seen this before)
(Other definitions for sound that I've seen before include "well" , "Healthy; narrow channel" , "to do the plumbing?" , "Ocean inlet or deep bay" , "Not diseased or damaged - listen" .)