Stiffener, for instance, taken over time (4)
I believe the answer is:
stay
'stiffener' is the definition.
(stay can mean an item stiffening a shirt collar)
'for instance taken over time' is the wordplay.
'for instance' becomes 'say' (both can be used to introduce an example).
'taken over' is an insertion indicator (some letters taken over or around others).
'time' becomes 't' (abbreviation).
'say' going around 't' is 'STAY'.
(Other definitions for stay that I've seen before include "Support, delay" , "Period in one place" , "Remain; steady" , "Stick around" , "Stanchion" .)