Cleaning brass? It’s an illegal activity (5,10)
I believe the answer is:
money laundering
'illegal activity' is the definition.
'cleaning brass? it's an' is the wordplay.
I cannot quite understand how this works, but
'cleaning' could be 'laundering' (laundering is a kind of cleaning) and 'laundering' is present in the answer.
'an' could be 'one' and 'one' is found within the answer.
The remaining letters 'my' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for money laundering that I've seen before include "Dishonest financial behaviour" , "crime" , "financial jiggery-pokery" .)