Working time's voluntary outside (11)
I believe the answer is:
operational
'working' is the definition.
(thesaurus)
'time's voluntary outside' is the wordplay.
'time' becomes 'era' (I've seen this before).
'voluntary' becomes 'optional' (I've seen this before).
'outside' indicates putting letters inside.
'era' going into 'optional' is 'OPERATIONAL'.
(Other definitions for operational that I've seen before include "working satisfactorily?" , "Ready for action" , "In working order" , "Ready for use" , "running" .)