Branch line from Waterloo, perhaps (4)
I believe the answer is:
loop
'branch line' is the definition.
(I've seen this before)
'from waterloo perhaps' is the wordplay.
'from' indicates a hidden word.
'LOOP' is hidden in the letters of 'waterloo perhaps'.
(Other definitions for loop that I've seen before include "Bight" , "going round" , "Shape produced by curve that crosses itself" , "Endless strip of tape" , "not informed one's out of it" .)