It stiffens a street-bridge (6)

I believe the answer is:

starch

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'it stiffens a' is the definition.
The definition suggests a singular noun which matches the answer.

'street-bridge' is the wordplay.
'street' becomes 'st' (abbreviation).
'bridge' becomes 'arch' (I've seen this before).
'st'+'arch'='STARCH'

Can you help me to learn more?

(Other definitions for starch that I've seen before include "Stiffening carbohydrate" , "Carbohydrate; stiffen" , "Stiffness of manner" , "Carbohydrate found in potatoes, wheat etc." , "foodstuff" .)

I've seen this clue in the Sydney Morning Herald.

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