Bygone era of individual in phase of development (5,3)
I believe the answer is:
stone age
'bygone era' is the definition.
'Stone Age' can be an answer for 'era' (thesaurus). I am not sure about the 'bygone' bit.
'individual in phase of development' is the wordplay.
I cannot quite see how this works, but
'individual' could be 'one' (an individual is one person) and 'one' is found within the answer.
'phase' could be 'stage' (synonyms) and 'stage' is located in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for stone age that I've seen before include "before copper came in" , "Early period" , "Earliest known period of human culture" , "Hard time" , "25 [ERA]" .)