Looking up a time zone (4)
I believe the answer is:
area
'zone' is the definition.
(I've seen this before)
'looking up a time' is the wordplay.
'looking up' is a reversal indicator (in a down clue, letters go up).
'time' becomes 'era' (an era is a period of time).
'a'+'era'='aera'
'aera' backwards is 'AREA'.
(Other definitions for area that I've seen before include "Patch" , "part of town" , "Length multiplied by breadth" , "Extent of a 2-dimensional surface" , "Part of a building, city etc" .)