Swan has key element (6)
I believe the answer is:
cobalt
'element' is the definition.
(I know that cobalt is a type of metal)
'swan has key' is the wordplay.
'swan' becomes 'cob' (cob is a kind of swan).
'has' is a charade indicator (letters next to each other).
'key' becomes 'alt' (the Alt keyboard button).
'cob'+'alt'='COBALT'
(Other definitions for cobalt that I've seen before include "Metal giving its name to type of deep blue" , "Deep blue colour" , "Silvery-white metal used in alloys" , "Metal element" , "given radiotherapy with this?" .)