In good condition (5)
'in good condition' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sound that I've seen before include "place for sailing, perhaps" , "Measure depth" , "stretch of water" , "Valid" , "Not diseased or damaged - listen" .)
(Other definitions for well-kept that I've seen before include "Carefully looked after" , "Carefully maintained" , "Shipshape" , "Maintained or supported in good condition" .)