Item of office equipment is essential, needed by firm's bosses (8)
I believe the answer is:
keyboard
'item of office equipment' is the definition.
(computer keyboard)
'essential needed by firm's bosses' is the wordplay.
'essential' becomes 'key' (key can mean essential or important).
'needed by' is a charade indicator (letters next to each other).
'firm's bosses' becomes 'board' (board of directors).
'key'+'board'='KEYBOARD'
'is' is the link.
(Other definitions for keyboard that I've seen before include "one lays hands on" , "Body rake (anag.)" , "Data entry device" , "(Part of) musical instrument" , "One produces notes" .)