Element used up in ignition (3)
I believe the answer is:
tin
'element' is the definition.
(tin is a kind of element)
'up in ignition' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' indicates the answer is hidden within the clue.
'nit' can be found hidden inside 'ignition'.
'nit' backwards is 'TIN'.
'used' is the link.
(Other definitions for tin that I've seen before include "Silvery-white metallic element" , "roofing material" , "Metal in Argentina" , "Ready" , "A silvery-white metal, symbol Sn" .)