Almost nothing about it, hard instrument (6)
I believe the answer is:
zither
'instrument' is the definition.
(zither is a kind of instrument)
'almost nothing about it hard' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'nothing' becomes 'zero' (synonyms).
'about it' means one lot of letters goes inside another.
'hard' becomes 'ith' (I can't justify this - if you can you should believe this answer much more).
'zero' with its final letter taken off is 'zer'.
'zer' enclosing 'ith' is 'ZITHER'.
Can you help me to learn more?
(Other definitions for zither that I've seen before include "Stringed box, played with plectrum" , "Harp-like instrument" , "Instrument played on the knee" , "Stringed instrument placed on the knee" , "Metal-stringed soundbox" .)