Non-mandatory (8)
I believe the answer is:
optional
'non-mandatory' is the definition.
(I know that non-mandatory can be written as optional)
This is all the clue.
(Other definitions for optional that I've seen before include "Not compulsory, discretionary" , "Too plain (anag.)" , "'Left to choice, not compulsory (8)'" , "Such as one can take or leave" , "'Elective, not obligatory (8)'" .)