Measures up narrow (4)
I believe the answer is:
slim
'narrow' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for slim that I've seen before include "Svelte" , "without much body" , "Annual horse race" , "Slender or slight" , "Elegantly thin or slender" .)