Press agent primarily recalled for one period of change (4,3)
I believe the answer is:
iron age
'period of change' is the definition.
'press agent primarily recalled for one' is the wordplay.
'press' becomes 'iron' (ironing is a kind of pressing).
'primarily' indicates taking the first letters.
'recalled' shows that the letters should be reversed in order.
'for one' becomes 'eg' (I've seen this in other clues).
The first letter of 'agent' is 'a'.
'eg' written backwards gives 'ge'.
'iron'+'a'+'ge'='IRON AGE'
(Other definitions for iron age that I've seen before include "Hist. cultural stage" , "A region (anag) - period long ago" , "It followed the Bronze Age" , "Period when metal tools were developed" , "Prehistoric period" .)