Pornographer firstly has a rule about body part (6)
I believe the answer is:
pleura
'body part' is the definition.
(pleura is a kind of body part)
'pornographer firstly has a rule about' is the wordplay.
'firstly' says to take the initial letters.
'has' means one lot of letters go next to another.
'about' indicates an anagram.
The initial letter of 'pornographer' is 'p'.
'a'+'rule'='arule'
'arule' is an anagram of 'leura'.
'p'+'leura'='PLEURA'
(Other definitions for pleura that I've seen before include "Membrane around the lungs" , "Membrane around lung" , "The membrane surrounding the lungs" .)