Street-bridge for a laundry additive (6)
I believe the answer is:
starch
'a laundry additive' is the definition.
(I've seen this in another clue)
'street-bridge' is the wordplay.
'street' becomes 'st' (abbreviation).
'bridge' becomes 'arch' (I've seen this before).
'st'+'arch'='STARCH'
'for' is the link.
(Other definitions for starch that I've seen before include "foodstuff" , "Carbohydrate found in potatoes, wheat etc." , "Formality - makes things stiff" , "Stuff that stiffens cloth" , "Stiffening agent" .)