Plant blocking extremely smart thoroughfare (6)
I believe the answer is:
street
'thoroughfare' is the definition.
(street is a kind of thoroughfare)
'plant blocking extremely smart' is the wordplay.
'plant' becomes 'tree' (tree is a kind of plant**).
'blocking' indicates putting letters inside.
'extremely' says to hollow out the word (remove centre letters).
'smart' with its middle removed is 'st'.
'tree' inserted inside 'st' is 'STREET'.
(Other definitions for street that I've seen before include "Way; setter (anag.)" , "City road" , "Tester in public road" , "Public road in a conurbation" , "Town thoroughfare" .)