Stable's number one horse one out of this world (6)
I believe the answer is:
spacer
'one out of this world' is the definition.
(space traveller)
'stable's number one horse' is the wordplay.
'stable's number one' becomes 's' (1st letter of 'stable').
'horse' becomes 'pacer' (a horse which paces).
's'+'pacer'='SPACER'
(Other definitions for spacer that I've seen before include "Something keeping apart" , "Thick washer" , "One keeping apart" , "One preserving gap" , "One may prevent physical contact" .)