Chicago Transit Authority trains (3)
I believe the answer is:
els
'chicago transit authority trains' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for els that I've seen before include "Successful owner of several clubs" , "Golfer; elevated railways" , "Angular distances above the horizon" , "Ernie - - (golfer)" , "US urban railways" .)