Not obligatory (8)
I believe the answer is:
optional
'not obligatory' is the definition.
('optional' is the opposite to 'obligatory')
This is all the clue.
(Other definitions for optional that I've seen before include "Too plain (anag.)" , "Elective, as you choose" , "Open to choice - take your pick" , "Platoon I join as a matter of choice" , "Available as a choice but not obligatory" .)