Long time inside after argument (3)
I believe the answer is:
era
'long time' is the definition.
(a long period of time)
'inside after argument' is the wordplay.
'inside' indicates a hidden word.
'ERA' is hidden within 'after argument'.
(Other definitions for era that I've seen before include "Age marked by a distinctive character" , "Aeon" , "Memorable period" , "quite some time" , "US equality law" .)