Stiffener used by most architects (6)
I believe the answer is:
starch
'stiffener' is the definition.
(starch is used to make fabric stiff)
'used by most architects' is the wordplay.
'used by' says the answer is hidden in the clue.
'STARCH' can be found hidden inside 'most architects'.
(Other definitions for starch that I've seen before include "It stiffens shirts and collars" , "Stuff that stiffens, say collars" , "Carbohydrate that occurs in all green plants" , "Stiffness; carbohydrate" , "Stiffening agent" .)