Elective, not obligatory (8)
I believe the answer is:
optional
I believe this clue is a double definition.
'elective' is the first definition.
(similar in meaning)
'not obligatory' is the second definition.
('optional' is the opposite to 'obligatory')
(Other definitions for optional that I've seen before include "Elective, as you choose" , "Too plain (anag.)" , "'Elective, not obligatory (8)'" , "Such as one can take or leave" , "Voluntary - not required" .)