In common parlance, it isn't dirty (5)
I believe the answer is:
taint
'dirty' is the definition.
(thesaurus)
'in common parlance it isn't' is the wordplay.
I cannot really see how this works, but
'in' is within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
The remaining letters 'at' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for taint that I've seen before include "Defect - contamination" , "Evidence of pollution" , "Contaminating influence" , "Contaminate, pollute" , "Source of infection" .)