Pass across start in buggy (7)
I believe the answer is:
transit
'pass across' is the definition.
(synonyms)
'start in buggy' is the wordplay.
'buggy' is an anagram indicator.
'start'+'in'='startin'
'startin' anagrammed gives 'TRANSIT'.
(Other definitions for transit that I've seen before include "In migration" , "Act of passing across" , "The act of carrying from one place to another" , "In - - (en route)" , "Conveyance of goods or people - tin star (anag)" .)