Subway rodent (3)
I believe the answer is:
rat
'subway rodent' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rat that I've seen before include "Rodent; informer (colloq.)" , "Abandon" , "Cad" , "Treacherous rodent" , "Domestic rodent" .)