Dual carriageway over sierra is most unnatural (9)
I believe the answer is:
strangest
'most unnatural' is the definition.
Both the answer and definition are superlative adjectives. Maybe there's a link between them I don't understand?
'dual carriageway over sierra' is the wordplay.
'dual carriageway' becomes 'stst' (I am not sure about this - if you are sure you should believe this answer much more).
'over' is an insertion indicator.
'sierra' becomes 'range' (I've seen this before).
'stst' enclosing 'range' is 'STRANGEST'.
'is' is the link.
Can you help me to learn more?
(Other definitions for strangest that I've seen before include "Most unusual or odd" , "Most remarkable" , "Most out of the ordinary and unexpected" , "Most unusual, oddest" , "Most peculiar" .)