It measures a short line, not the whole underground (5)

I believe the answer is:

meter

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

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

I don't understand the remainder of the clue.

(Other definitions for meter that I've seen before include "Measuring device, say for electricity usage" , "Apparatus for measuring" , "Gauge" , "Instrument for measuring quantity" , "Something measuring" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.