Working day kept by choice (9)
I believe the answer is:
operation
'working' is the definition.
(how something operates)
'day kept by choice' is the wordplay.
'day' becomes 'era' (day can mean an era of time).
'kept by' means one lot of letters goes inside another.
'choice' becomes 'option' (synonyms).
'era' placed inside 'option' is 'OPERATION'.
(Other definitions for operation that I've seen before include "In hospital procedure" , "To one pair (anag.)" , "medical treatment" , "Handling" , "working" .)