Far-out rock instrument unboxed in post office (5)
I believe the answer is:
pluto
'far-out rock' is the definition.
The definition and answer can be both natural objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'instrument unboxed in post office' is the wordplay.
'instrument' becomes 'lute' (lute is a kind of instrument).
'unboxed' means to look at the middle letters (middle letters are unwrapped or unboxed).
'in' indicates putting letters inside.
'post office' becomes 'plo' (I am not sure about this - if you are sure you should believe this answer much more).
The central letters of 'lute' are 'ut'.
'ut' placed within 'plo' is 'PLUTO'.
Can you help me to learn more?
(Other definitions for pluto that I've seen before include "Distant planet" , "Infernal ruler" , "it's another world there" , "heavenly body" , "Planet furthest from the sun" .)