Long time inside after Altamont (3)
I believe the answer is:
era
'long time' is the definition.
(a long period of time)
'inside after altamont' is the wordplay.
'inside' says the answer is hidden in the clue.
'ERA' is hidden in the letters of 'after altamont'.
(Other definitions for era that I've seen before include "Epoch, period" , "US equality law" , "Important time" , "Significant time" , "Characteristic period" .)