Boundary made by iron-clad circle (6)
I believe the answer is:
fringe
'boundary' is the definition.
(fringe is a kind of boundary)
'iron-clad circle' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'clad' indicates putting letters inside.
'circle' becomes 'ring' (synonyms).
'fe' enclosing 'ring' is 'FRINGE'.
'made by' is the link.
(Other definitions for fringe that I've seen before include "Rim" , "Border; not mainstream" , "Thrum" , "These benefits are additional to pay" , "lunatics, perhaps" .)