Chart-topper, a little offensive (7)
I believe the answer is:
noisome
'offensive' is the definition.
(I know that offensive can be written as noisome)
'chart-topper a little' is the wordplay.
'chart-topper' becomes 'No I' (resembles No. 1, top of the chart).
'a little' becomes 'some' (both can mean a small amount).
'noi'+'some'='NOISOME'
(Other definitions for noisome that I've seen before include "Odorous" , "Repugnant" , "Disagreeable, unpleasant" , "Obnoxious" , "Harmful to health; disgusting" .)