Reflecting, for example, on a period of history (3)
I believe the answer is:
age
'period of history' is the definition.
(I've seen this before)
'reflecting for example on a' is the wordplay.
'reflecting' is a reversal indicator.
'for example' becomes 'eg' ('e.g.' can be a synonym of 'for example').
'on' means one lot of letters go next to another.
'eg'+'a'='ega'
'ega' reversed gives 'AGE'.
(Other definitions for age that I've seen before include "How long something has existed" , "16/18 [INDUSTRIAL REVOLUTION]" , "Historic period" , "Just a number" , "Mature, grow in years" .)