Unconventional start in passage (7)
I believe the answer is:
transit
'passage' is the definition.
('transit' can be a synonym of 'passage')
'unconventional start in' is the wordplay.
'unconventional' indicates anagramming the letters.
'start'+'in'='startin'
'startin' anagrammed gives 'TRANSIT'.
(Other definitions for transit that I've seen before include "Haulage" , "Movement (of goods)" , "Passage of a planet across the sun's disc" , "Act of passing across" , "Passage of heavenly body" .)