Railway track support (7)
I believe the answer is:
sleeper
'railway track support' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sleeper that I've seen before include "Railway track support" , "One who's out" , "Beam across railway track or berth on ship" , "Brace to support rail on track" , "Earring" .)