Choosing to save time working (9)
I believe the answer is:
operating
'working' is the definition.
('operate' can be a synonym of 'work')
'choosing to save time' is the wordplay.
'choosing' becomes 'opting' (I've seen this before).
'to save' means one lot of letters goes inside another.
'time' becomes 'era' (I've seen this before).
'opting' placed around 'era' is 'OPERATING'.
(Other definitions for operating that I've seen before include "Working" , "Directing or controlling, machinery say" , "Cutting" , "Active" , "Functioning" .)