Blue rant in paper's leader unfortunately too rude to publish ... (11)
I believe the answer is:
unprintable
'too rude to publish' is the definition.
I can't tell whether this definition defines the answer.
'blue rant in paper's leader unfortunately' is the wordplay.
'leader' indicates taking the first letters.
'unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
The initial letter of 'papers' is 'p'.
'blue'+'rant'+'in'+'p'='bluerantinp'
'bluerantinp' with letters rearranged gives 'UNPRINTABLE'.
Can you help me to learn more?
(Other definitions for unprintable that I've seen before include "Not fit to be read - potentially libellous?" , "Too shocking to be published" , "Not fit for publication" , "rejected by editor" .)