Essential to get on a musical instrument (8)
I believe the answer is:
keyboard
'a musical instrument' is the definition.
(I've seen this before)
'essential to get on' is the wordplay.
'essential' becomes 'key' (I've seen this before).
'to get on' becomes 'board' ('board' can be a synonym of 'get on').
'key'+'board'='KEYBOARD'
(Other definitions for keyboard that I've seen before include "Piano or computer device" , "one lays hands on" , "Data entry device" , "facility for writing" , "(Part of) musical instrument" .)