Decorative chain having two metals additionally inset (7)
I believe the answer is:
festoon
'decorative chain' is the definition.
(I've seen this before)
'two metals additionally inset' is the wordplay.
'two metals' means two replacements for 'metal'.
'metal' becomes 'Fe' (Fe is the atomic symbol for iron).
'metal' becomes 'sn' (Sn is the atomic symbol for tin).
'additionally' becomes 'too' (I've seen this before).
'inset' indicates putting letters inside (some letters are set inside others).
'sn' enclosing 'too' is 'stoon'.
'fe'+'stoon'='FESTOON'
'having' acts as a link.
(Other definitions for festoon that I've seen before include "Garland with flowers" , "swag" , "So often (anag.)" , "Decorate with hanging ornaments" , "Adorn (with garlands?)" .)