Edge hoop in iron (6)
I believe the answer is:
fringe
'edge' is the definition.
(I know that fringe is a type of edge)
'hoop in iron' is the wordplay.
'hoop' becomes 'ring' (synonyms).
'in' is an insertion indicator.
'iron' becomes 'fe' (chemical symbol for iron).
'ring' inserted inside 'fe' is 'FRINGE'.
(Other definitions for fringe that I've seen before include "lunatics, perhaps" , "Border, outer part" , "Rim" , "Perimeter" , "Periphery" .)