Road test failing on plugs (6)
I believe the answer is:
street
'road' is the definition.
(I know that street is a type of road)
'test failing on plugs' is the wordplay.
'failing' is an anagram indicator (a failed spelling of the word).
'on' becomes 're' ('on' can be mean 'with reference to').
'plugs' is an insertion indicator (plug can mean to fill a gap).
'test' is an anagram of 'stet'.
'stet' placed around 're' is 'STREET'.
(Other definitions for street that I've seen before include "Urban road" , "City thoroughfare" , "Paved road" , "Roadway" , "Form of theatre" .)