… leading politician switching motorway to south — very worrying (8)
I believe the answer is:
sinister
'south very worrying' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I can't see?
'leading politician switching motorway' is the wordplay.
I cannot really see how this works, but
'leading' could be 'in' (in government) and 'in' is located in the answer.
The remaining letters 'sister' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'to' is the link.
(Other definitions for sinister that I've seen before include "Menacing" , "Threatening - ominous" , "'Ominous, baleful (8)'" , "Disturbing" , "Suggestive of evil, threatening harm" .)