Essential to bring up son's pornography (4)

I believe the answer is:

smut

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'pornography' is the definition.
(I know that pornography can be written as smut)

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for smut that I've seen before include "Obscene word(s)" , "Small flake of dirt" , "Soot flake or sexual innuendo" , "Indecent items or talk" , "Smudge of soot" .)

I've seen this clue in The Guardian.

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