Roman street artist swallows good drug (6)
I believe the answer is:
viagra
'drug' is the definition.
(I have seen 'Potency drug ' mean 'viagra' so perhaps 'drug' could also mean 'viagra')
'roman street artist swallows good' is the wordplay.
'roman street' becomes 'via' (Italian word for a street).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'swallows' is an insertion indicator.
'good' becomes 'g' (abbreviation).
'via'+'ra'='viara'
'viara' going around 'g' is 'VIAGRA'.
(Other definitions for viagra that I've seen before include "Performance-enhancing drug?" , "Male sexual aid" , "potent drug?" , "Promoter of the other" , "Drug aiding one's sex life" .)