Lines like the London Overground (3)
I believe the answer is:
els
'lines like the london overground' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for els that I've seen before include "Golfer; elevated railways" , "Angular distances above the horizon" , "US urban railways" , "Successful owner of several clubs" , "Ernie - - (golfer)" .)