Little devil, one obtaining exemption from punishment (8)
I believe the answer is:
impunity
'exemption from punishment' is the definition.
(I've seen this in another clue)
'little devil one' is the wordplay.
'little devil' becomes 'imp' (an imp is a little demon).
'one' becomes 'unity' (synonyms).
'imp'+'unity'='IMPUNITY'
'obtaining' is the link.
(Other definitions for impunity that I've seen before include "Complete protection from unpleasant consequences" , "Exemption from punishment or loss" , "Freedom from punishment" , "Rascal, by agreement, gets exemption" , "Not subject to the normal consequences" .)