Journey there and back on the circle line?
I believe the answer is:
round trip
'journey there and back' is the definition.
(I've seen this before)
'the circle line?' is the wordplay.
'the circle' becomes 'round' ('round' can be a synonym of 'circle').
'line?' becomes 'trip' (trip line is a kind of line**).
'round'+'trip'='ROUND-TRIP'
'on' acts as a link.
(Other definitions for round trip that I've seen before include "Journey returning to start" , "collecting drinks from the bar?" , "Return journey" , "bar and back?" , "kind of voyage" .)