An element of one free, one … er … (7)
I believe the answer is:
iridium
'an element' is the definition.
(iridium is a kind of element)
'one free one er' is the wordplay.
'one' becomes 'i' (Roman numeral).
'free' becomes 'rid' (to be free of something is to be rid of it).
'one' becomes 'i' (Roman numeral).
'er' becomes 'um' (both are expressions of uncertainty).
'i'+'rid'+'i'+'um'='IRIDIUM'
'of' acts as a link.
(Other definitions for iridium that I've seen before include "Silvery-white metal" , "Heavy corrosion-resistant metal, used in alloys" , "Heavy brittle metal used in alloys, Ir" , "Corrosion inhibitor" , "Hard metal element, Ir" .)