Monorails, often (3)
I believe the answer is:
els
'monorails often' 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 "Successful owner of several clubs" , "Angular distances above the horizon" , "US urban railways" , "Ernie - - (golfer)" , "South African golfer" .)