Era when men began forging ahead (4,3)
I believe the answer is:
iron age
'era' is the definition.
(I have seen 'bygone era' mean 'iron age' so perhaps 'era' could also mean 'iron age')
'men began forging ahead' is the wordplay.
I cannot quite see how this works, but
'ahead' could be 'on' (synonyms) and 'on' is present in the answer.
This explanation may well be incorrect...
'when' acts as a link.
Can you help me to learn more?
(Other definitions for iron age that I've seen before include "prehistoric times" , "Hist. cultural stage" , "Prehistoric period" , "bygone era" , "A region (anag) - period long ago" .)