Essential, indispensable (9)
I believe the answer is:
necessary
This clue is a double definition.
'essential' is the first definition.
(I know that essential can be written as necessary)
'indispensable' is the second definition.
(associated in meaning)
(Other definitions for necessary that I've seen before include "Required, needed" , "Indispensable, essentional" , "Inescapable" , "Vital" , "Required to be done" .)