Girl seen on Circle line? (5)
I believe the answer is:
lasso
'line?' is the definition.
(lasso is a kind of line)
'girl seen on circle' is the wordplay.
'girl' becomes 'lass' (lass is a kind of girl**).
'seen on' is a charade indicator (letters next to each other).
'circle' becomes 'o'.
'lass'+'o'='LASSO'
(Other definitions for lasso that I've seen before include "Belgian composer" , "A cowboy's rope with a running noose" , "Rope with 20" , "Rope for catching livestock" , "Cowcatcher" .)