Someone getting on Circle line by exit (5)
I believe the answer is:
oldie
'someone getting on' is the definition.
(get on can mean to get older)
'circle line by exit' is the wordplay.
'circle' becomes 'O' (O is a circle).
'line' becomes 'l' (used when specifying particular lines from a poem).
'by' is a charade indicator (letters next to each other).
'exit' becomes 'die' (synonyms).
'o'+'l'+'die'='OLDIE'
(Other definitions for oldie that I've seen before include "Popular song from the past" , "Veteran" , "forties film, say" , "one who is past it?" , "Aged person (colloq.)" .)