Loaf, key element (6)
I believe the answer is:
cobalt
'element' is the definition.
(cobalt is a kind of element)
'loaf key' is the wordplay.
'loaf' becomes 'cob' (I've seen this before).
'key' becomes 'alt' (I've seen this in other clues).
'cob'+'alt'='COBALT'
(Other definitions for cobalt that I've seen before include "Magnetic-alloy element; a blue" , "Sort of blue" , "Metallic element used in blue/green pigments" , "Silvery-white metal used in alloys" , "Metal element" .)