Space on bus is potential road home (8)
I believe the answer is:
headroom
'space on bus' is the definition.
'potential road home' is the wordplay.
'potential' indicates anagramming the letters.
'road'+'home'='roadhome'
'roadhome' anagrammed gives 'HEADROOM'.
'is' is the link.
(Other definitions for headroom that I've seen before include "Clearance vertically" , "Space available above" , "Clearance below a bridge or ceiling" , "vertical space" , "enough space" .)