Most frightening sign of injury that is seen on street (8)
I believe the answer is:
scariest
'most frightening' is the definition.
(I've seen this before)
'sign of injury that is seen on street' is the wordplay.
'sign of injury' becomes 'scar' (I've seen this before).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'seen on' means one lot of letters go next to another.
'street' becomes 'st'.
'scar'+'ie'+'st'='SCARIEST'
(Other definitions for scariest that I've seen before include "Most frightening" , "Most terrifying" , "most alarming" .)