The most common and most likely answer to this crossword clue is the 5 plus 8 letter word LINDA LOVELACE
I've seen this crossword clue in The Guardian
L
I
N
D
A
L
O
V
E
L
A
C
E
Buddy explains!
The straight part
We know that "Porn star" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case porn star = linda lovelace.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.