For instance, a revolutionary period (3)
I believe the answer is:
age
'period' is the definition.
(age is a kind of period)
'for instance a revolutionary' is the wordplay.
'for instance' becomes 'eg' (synonyms).
'revolutionary' says the letters should be written backwards.
'eg'+'a'='ega'
'ega' backwards is 'AGE'.
(Other definitions for age that I've seen before include "Long period" , "Time in years" , "Era; grow old" , "Mature, grow in years" , "Period of history" .)