Gets on when there's no time to get in (4)
I believe the answer is:
ages
'gets on' is the definition.
(get on can mean to get older)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ages that I've seen before include "Epochs" , "Long periods" , "Rock of - - (hymn)" , "Donkey's years" , "Indefinitely long time" .)