Ace in race vehicle getting measure of Americans (5)

I believe the answer is:

karat

'measure of americans' is the definition.
(US spelling of carat)

'ace in race vehicle' is the wordplay.
'ace' becomes 'a'.
'in' is an insertion indicator.
'race vehicle' becomes 'kart'.
'a' going inside 'kart' is 'KARAT'.

'getting' acts as a link.

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app