It's indecent seeing mother use these openers (4)
I believe the answer is:
smut
'it's indecent' is the definition.
I can't tell whether this defines the answer.
'seeing mother use these openers' is the wordplay.
'openers' indicates taking the first letters.
The initial letters of 'seeing mother use these' is 'SMUT'.
Can you help me to learn more?
(Other definitions for smut that I've seen before include "Dirt or immoral talk" , "Flake of soot or dirt" , "Mildly obscene remarks" , "Bit of soot or dirty talk" , "Soot; obscenities" .)