Time zone reduced after upset (3)
I believe the answer is:
era
'time' is the definition.
(era is a kind of time)
'zone reduced after upset' is the wordplay.
'zone' becomes 'area' (I've seen this before).
'reduced' means to remove the last letter.
'after upset' says the letters should be written backwards (in a down clue, letters go up).
'area' with its last letter removed is 'are'.
'are' backwards is 'ERA'.
(Other definitions for era that I've seen before include "Memorable period" , "Period, age" , "Distinctive period of time" , "Long period of history" , "Distinct period in history" .)