Running is not compulsory, taking in a long period (11)
I believe the answer is:
operational
'running' is the definition.
(I've seen this before)
'not compulsory taking in a long period' is the wordplay.
'not compulsory' becomes 'optional' (I've seen this before).
'taking in' is an insertion indicator.
'a long period' becomes 'era' (I've seen this before).
'optional' enclosing 'era' is 'OPERATIONAL'.
'is' is the link.
(Other definitions for operational that I've seen before include "running" , "Ready for action" , "In working order" , "Ready for use" , "working satisfactorily?" .)