Element that's key on horse (6)
I believe the answer is:
cobalt
'element' is the definition.
(cobalt is a kind of element)
'key on horse' is the wordplay.
'key' becomes 'alt' (I've seen this in other clues**).
'on' is a charade indicator (letters next to each other).
'horse' becomes 'cob' (cob is a kind of horse).
'alt' put after 'cob' is 'COBALT'.
'that's' acts as a link.
(Other definitions for cobalt that I've seen before include "Element 27; a deep blue" , "Deep shade of 16 dn" , "given radiotherapy with this?" , "Deep blue colour" , "Silvery-white metal used in alloys" .)