Rail support (7)
I believe the answer is:
sleeper
'rail support' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sleeper that I've seen before include "One having D [DREAM]" , "Rail supporter; secret agent" , "Unactivated spy - railway support" , "Beam across railway track or berth on ship" , "latent spy" .)