Before noon, peer amended current measure (6)
I believe the answer is:
ampere
'current measure' is the definition.
(I've seen this in another clue)
'before noon peer amended' is the wordplay.
'before noon' becomes 'am'.
'amended' indicates an anagram.
'peer' anagrammed gives 'pere'.
'am'+'pere'='AMPERE'
(Other definitions for ampere that I've seen before include "Current measure" , "scientist" , "Basic SI unit of electric current" , "Current analyst" , "Unit of currency" .)