Not essential (10)
'not essential' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for extraneous that I've seen before include "Foreign" , "Irrelevant; of outside origin" , "Of external origin" , "Unrelated, irrelevant" , "Not belonging" .)
(Other definitions for expendable that I've seen before include "Easily abandoned" , "Easily sacrificed" , "not important" , "Able to be sacrificed" .)
(Other definitions for dispensable that I've seen before include "Non-essential" , "Able to be done without" , "That can be got rid of" .)