What's penned by Diderot — an essay about lawmaker (7)
I believe the answer is:
senator
'lawmaker' is the definition.
(senator is a kind of lawmaker)
'what's penned by diderot an essay about' is the wordplay.
'what's penned by' indicates the answer is hidden within the clue.
'about' says the letters should be written backwards.
'rotanes' can be found hidden inside 'diderot an essay'.
'rotanes' reversed gives 'SENATOR'.
(Other definitions for senator that I've seen before include "Treason committed by member of Upper House" , "Politician involved in treason" , "Treason by member of parliament" , "Member of Upper House engaged in treason" , "Roman politician" .)