Dog that is revealing measure of radioactivity (5)
I believe the answer is:
curie
'measure of radioactivity' is the definition.
(I've seen this before)
'dog that is' is the wordplay.
'dog' becomes 'cur' (both can mean a despicable person).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'cur'+'ie'='CURIE'
'revealing' acts as a link.
(Other definitions for curie that I've seen before include "Marie ---, found radium" , "Marie -, Fr. physicist" , "Unit of radioactivity" , "Scientist" , "Pioneer in radioactivity" .)