Measurement across tier made in error (8)
I believe the answer is:
diameter
'measurement' is the definition.
(diameter is a kind of measurement)
'across tier made in error' is the wordplay.
'across' says to put letters next to each other (across can mean on).
'in error' is an anagram indicator.
'tier' after 'made' is 'madetier'.
'madetier' anagrammed gives 'DIAMETER'.
(Other definitions for diameter that I've seen before include "A demerit for line bisecting a circle" , "Maximum distance across circle" , "Straight line across circle passing through its centre" , "It crosses the circle at its widest" , "Line that crosses the centre of a circle" .)