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' suggests taking the first letters.
'has' says to put letters next to each other.
'about' is an anagram indicator.
The initial letter of 'pornographer' is 'p'.
'a'+'rule'='arule'
'arule' with letters rearranged gives '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" .)