It is not coarse or dirty (5)
I believe the answer is:
taint
'dirty' is the definition.
(thesaurus)
'it is not' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'is not' becomes 'aint' (non-standard version of 'isn't').
't'+'aint'='TAINT'
'coarse or' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for taint that I've seen before include "Stigmatise" , "Defile" , "Contaminate, pollute" , "Blemish" , "Defect - contamination" .)