Current measuring device missing small top (6)

I believe the answer is:

extant

I'm a little stuck... Click here to teach me more about this clue!

'current' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for extant that I've seen before include "Next at being still existing" , "remaining" , "Existing, alive" , "left" , "still standing" .)

I've seen this clue in The Irish Times.

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