Cosmetic treatment of soilpipe so 'off' (6)
I believe the answer is:
lippie
'cosmetic treatment' is the definition.
The definition suggests a singular noun which matches the answer.
'of soilpipe so off' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'off' is a deletion indicator (some letters taken off).
'soilpipe' with 'so' taken out is 'ilpipe'.
'ilpipe' is an anagram of 'LIPPIE'.
Can you help me to learn more?
(Another definition for lippie that I've seen is " make-up".)