Subway station (4)
I believe the answer is:
stop
'subway station' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stop that I've seen before include "Plug - organ knob - bus halt - period" , "Cease, desist, whoa" , "Part of organ" , "End or pause" , "high card in bridge" .)