Plain staff that is inset with fine stone (8)
I believe the answer is:
manifest
'plain staff that' is the definition.
'manifest' can be an answer for 'plain' (thesaurus). I am not sure about the remainder of the definition.
'inset with fine stone' is the wordplay.
'inset' becomes 'mast' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' means one lot of letters goes inside another.
'stone' indicates an anagram (I've seen 'stoned' mean this (can mean on drugs or drunk)).
'fine' anagrammed gives 'nife'.
'mast' going around 'nife' is 'MANIFEST'.
'is' is the link.
Can you help me to learn more?
(Other definitions for manifest that I've seen before include "Plain - cargo list" , "Obvious to the eye" , "Display" , "Ship's document; plain" , "List of ship's cargo or passengers" .)