Working is a matter of choice - about time! (11)
I believe the answer is:
operational
'working' is the definition.
(I've seen this in another clue)
'a matter of choice about time' is the wordplay.
'a matter of choice' becomes 'optional' (I've seen this before).
'about' means one lot of letters goes inside another.
'time' becomes 'era' (era is a kind of time).
'optional' going around 'era' is 'OPERATIONAL'.
'is' acts as a link.
(Other definitions for operational that I've seen before include "Ready for action" , "Ready for use" , "running" , "working satisfactorily?" , "In working order" .)