In good condition (4-4)
'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" .)