Musical box: relic spattered with mud (8)
I believe the answer is:
dulcimer
'musical box' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'relic spattered with mud' is the wordplay.
'spattered with' is an anagram indicator.
'relic' anagrammed with 'mud' makes 'DULCIMER'.
Can you help me to learn more?
(Other definitions for dulcimer that I've seen before include "Hammered instrument" , "one hit with hammer" , "Percussion instrument struck with small hammers" , "Something instrumental" , "Musical instrument - lime curd (anag)" .)