One replacing a string? Not on this instrument (8)
I believe the answer is:
recorder
'one replacing a string? not on this instrument' is the definition.
'recorder' can be an answer for 'instrument' (recorder is a kind of instrument). I am not certain of the remainder of the definition.
'string? not on this instrument' is the wordplay.
'string? not' becomes 'rec' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'this instrument' becomes 'order' (order is a kind of instrument).
'rec'+'order'='RECORDER'
Can you help me to learn more?
(Other definitions for recorder that I've seen before include "Fipple-flute" , "Judge - musical instrument" , "Wind instrument - historian, perhaps" , "A kind of flute or note-taker" , "sound device" .)