Rogue is mighty puckish, originally (3)
I believe the answer is:
imp
'rogue' is the definition.
(thesaurus)
'is mighty puckish originally' is the wordplay.
'originally' suggests taking the first letters.
The initial letters of 'is mighty puckish' is 'IMP'.
(Other definitions for imp that I've seen before include "puck" , "old graft" , "Emperor" , "Naughty demon" , "Evil spirit" .)