Some era — generally a long period (3)
I believe the answer is:
age
'long period' is the definition.
(I've seen this in another clue)
'some era generally a' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'AGE' is hidden in the letters of 'era generally a'.
(Other definitions for age that I've seen before include "Get older" , "Grow older" , "Years since birth" , "Maturity" , "Length of life" .)