One metre, for example (4)
I believe the answer is:
unit
This clue is a double definition.
'one' is the first definition.
(synonyms)
'metre for example' is the second definition.
(metre is a kind of unit)
(Other definitions for unit that I've seen before include "A single, complete item" , "Subdivision of a military group" , "Individual component" , "Single thing or person" , "Single item, maybe part of a set" .)