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