Left to choice, not compulsory (8)
I believe the answer is:
optional
I believe this is a double definition.
'left to choice' is the first definition.
(I've seen this before)
'not compulsory' is the second definition.
(I've seen this in another clue)
(Other definitions for optional that I've seen before include "something extra?" , "Voluntary - not required" , "Such as one can take or leave" , "Too plain (anag.)" , "'Left to choice, not compulsory (8)'" .)