Way fish harden (5)
I believe the answer is:
steel
'harden' is the definition.
(thesaurus)
'way fish' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'fish' becomes 'eel' (eel is a kind of fish).
'st'+'eel'='STEEL'
(Other definitions for steel that I've seen before include "Iron/carbon alloy" , "Determination, metal" , "Hard iron alloy" , "brace" , "Mixture of carbon and iron" .)