Found black stuff in the street (5)
I believe the answer is:
start
'found' is the definition.
(as in to establish)
'black stuff in the street' is the wordplay.
'black stuff' becomes 'tar'.
'in the' indicates putting letters inside.
'street' becomes 'st'.
'tar' inserted into 'st' is 'START'.
(Other definitions for start that I've seen before include "Move quickly < when surprised" , "Take the first step" , "get under way" , "Begin; flinch" , "Commencement" .)