Hoop in iron shows edge (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' put within 'fe' is 'FRINGE'.
'shows' is the link.
(Other definitions for fringe that I've seen before include "Thrum" , "Tasselled edging" , "(Tasselled) border" , "Periphery" , "Finger (anag.)" .)