Racecourse favourite Star must step on it? (3,6)
I believe the answer is:
red carpet
'star must step on it?' is the definition.
(film stars walk the red carpet)
'racecourse favourite' is the wordplay.
'racecourse' becomes 'redcar' (racecourse in North Yorkshire).
'favourite' becomes 'pet' (synonyms).
'redcar'+'pet'='RED CARPET'
(Other definitions for red carpet that I've seen before include "What a distinguished visitor walks on" , "sort of deferential treatment?" , "sort of treatment for special visitors?" , "It's rolled out for special guests" , "It is rolled out for VIPs" .)