Gets on and leaves top off (4)
I believe the answer is:
ages
'gets on' is the definition.
(get on can mean to get older)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ages that I've seen before include "A long time (colloq.)" , "Gets older" , "Epochs" , "Rock of - - (hymn)" , "Historical eras" .)