Equality of measure some recognised in one test (8)
I believe the answer is:
isometry
'equality of measure' is the definition.
(I've seen this before)
'some recognised in one test' is the wordplay.
'recognised in' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'test' becomes 'try' (synonyms).
'i'+'try'='itry'
'some' going within 'itry' is 'ISOMETRY'.
(Other definitions for isometry that I've seen before include "mathematical concept" , "Equality of measure" , "all being of equal stature" , "equal measure" , "equality of proportion" .)