Gold plating ultimately added onto an instrument (5)
I believe the answer is:
organ
'instrument' is the definition.
(I know that organ is a type of electronic instrument)
'gold plating ultimately added onto an' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'ultimately' says to take the final letters.
'added' is a charade indicator (letters next to each other).
'onto' means one lot of letters go next to another.
The final letter of 'plating' is 'g'.
'or'+'g'+'an'='ORGAN'
(Other definitions for organ that I've seen before include "Air-powered instrument" , "Possibly stomach" , "Brain, for example - instrument" , "ear maybe" , "Perhaps one that's stopped" .)