Early period (4)
(Other definitions for iron age that I've seen before include "Hist. cultural stage" , "prehistoric times" , "Ancient period in the development of mankind" , "A region (anag) - period long ago" , "Time of the spread of metal weapons, tools etc" .)
'early period' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for morn that I've seen before include "PM? No" , "Early part of day (poetic)" , "First part of day (poet.)" , "Day's first part(poetic)" .)