Musical instrument, essential, found with committee (8)
I believe the answer is:
keyboard
'musical instrument' is the definition.
(I know that keyboard is a type of musical instrument)
'essential found with committee' is the wordplay.
'essential' becomes 'key' (key can mean essential or important).
'found with' is a charade indicator (letters next to each other).
'committee' becomes 'board' (board is a kind of committee).
'key'+'board'='KEYBOARD'
(Other definitions for keyboard that I've seen before include "Part of piano. computer, etc" , "Component of piano" , "Body rake (anag.)" , "Type" , "Piano or computer device" .)