Metal heel: one secured by old lady (7)
I believe the answer is:
cadmium
'metal' is the definition.
(cadmium is a kind of metal)
'heel one secured by old lady' is the wordplay.
'heel' becomes 'cad' ('cad' can be a synonym of 'heel').
'one' becomes 'i' (Roman numeral).
'secured by' is an insertion indicator (I've seen this in other clues).
'old lady' becomes 'mum' (I've seen this before).
'i' placed within 'mum' is 'mium'.
'cad'+'mium'='CADMIUM'
(Other definitions for cadmium that I've seen before include "Soft bluish-white metallic element" , "Toxic metal used in plating" , "Metallic element, Cd" , "Metal resembling zinc" , "cell" .)