Electric guitar ultimately has key objective of getting louder (9)
I believe the answer is:
crescendo
'getting louder' is the definition.
(I've seen this before)
'electric guitar ultimately has key objective of' is the wordplay.
'ultimately' indicates one should take the final letters.
'has' says to put letters next to each other.
'key' becomes 'esc' ('Escape' key on a computer keyboard**).
'objective' becomes 'end' (I've seen this before).
'of' becomes 'o'.
The last letters of 'electric guitar' are 'cr'.
'cr'+'esc'+'end'+'o'='CRESCENDO'
(Other definitions for crescendo that I've seen before include "hear this from crowd?" , "Gradual loudening" , "Volume getting larger" , "with score building" , "Intensify" .)