Change, once, from half marathon, adopting 2000m? (6)
I believe the answer is:
markka
'change once' is the definition.
(former Finnish coinage)
'half marathon adopting 2000m?' is the wordplay.
'half' indicates taking half.
'adopting' is an insertion indicator.
'2000m?' becomes 'kk' (two kilometres).
'marathon' cut in half is 'mara'.
'mara' placed around 'kk' is 'MARKKA'.
'from' is the link.
(Other definitions for markka that I've seen before include "Former Finnish currency unit" , "Ready once" .)